And that's not even the largest ship!note Any resemblance to four 1/25 scale engine blocks glued together is purely unavoidable.
Your starship is big, very big. Now how do you get this across to people? By stating how big it is in miles or kilometers. It may be a Starship Luxurious or so packed full of people or machinery that the interior resembles that of a submarine. Generally, these ships are too massive to land, so they're built in space and stay in space.
Often, mile-long starships are considered to be "harder" than small ones, partially because a slower-than-light ship would need to be huge to carry all the fuel and supplies needed for a decades-long voyage.
The most realistic designs—the ones that are very clear how lethal most high-powered spaceship engines are—tend to be very long, specifically, in order to maximize distance between the habitat section and the engine. In order to minimize mass (since mass is proportional to volume and one dimension is already determined), long ships will often be very thin.
Supertrope to Planet Spaceship and Unnecessarily Large Vessel. See also: Cool Starship, Big Dumb Object, The Battlestar and Generation Ship.
The Macross is a city-sized alien spaceship which crash lands on Earth, which is then used by humans to go into space. 1210 meters (3970 feet) long.
The assorted sequel series introduce the New Macross Class colonisation ships separated into 'Battle' sections that are at least the same approximate size of the original SDF-1, attached to massive 'City' sections which are exactly what the name suggests, massive, space going cities.
Zentraedi ships are even bigger, since their crews are 9 meter giants (except when they undergo "micronization" to human-size). A Nupetiet-Vergnitzs flagship, for instance, is about 4 kilometers long, and a Thuverl-Salan cruiser is 2300 meters.
In Gunbuster the Earth evacuation ship-turned-warship Eltreum is said to be 70 km long.
Mobile Suit Gundam 00 features the Celestial Being mothership, which measures roughly 15 km in length. (Though not technically "ships" per-se, space colonies in all Gundam series that feature them are roughly 5 miles in length.)
The UNSC and covenant ships in Mass Effect Clash Of Civilizations, whose scale astounds the Council team sent to investigate them, especially the Covenant supercarrier they dub "the Leviathan".
Star Wars: The infamous Imperial-class Star Destroyers are 1,600 meters long, or approximately one mile, while Darth Vader's Super Star Destroyer Executor was 19 kilometers. However, the Venator-classes shown in Revenge of the Sith were only 1,137 meters and apparently capable of landing. The Star Wars Expanded Universe adds many more examples, some of which are found under Literature. The Galactic Empire has the justification of the Tarkin Doctrine: a Mile-Long Ship is imposing and scary and symbolic of the Empire's might. Certain Rebel ships match or exceed the ISD in length: Admiral Ackbar's flagship Home One is nearly four kilometers, though as Mon Calamari Star Cruisers are custom-built they vary pretty widely in length.
Spaceball One in the movie Spaceballs, a parody of Star Wars, has a similar scale, justified by the fact that it needs to be capable of storing the entire atmosphere of an Earth-like planet. The movie opens with a shot of the ship flying past the camera — and it takes almost two minutes to pass, with the shot functioning as an Overly-Long Gag. There's also a number of jokes about how big the ship is, such as:
The opening scene of Saturn 3 features an enormous spaceship, meant to visit outposts on various moons and planets in the solar system at sub-light speeds.
The ISV Venture Star in Avatar is stated in some sources to be about 1500 meters long (almost a mile). The same is true for its 11 sister ships of the Capital Star class. In fact, the length is considered ''small' and is only possible with the use of Unobtainium. The original ship sent from Earth to Alpha Centauri was 4 kilometers (2.5 miles) long in order to make room for massive refrigeration units for the magnets keeping Anti Matter contained.
In Stephen Baxter's Xeelee Sequence universe: the Xeelee Nightfighter. The cockpit is small, about the size of a room, but its wings stretch out for kilometers in either direction, like vast sails. The purpose of these wings are never made evident, as the ship itself travels via teleportation.
E. E. “Doc” Smith's book Skylark of Valeron: the Skylark was a sphere over 1000km in radius and has a crew of four, it needed to be that big to contain the navigational instruments necessary to cross intergalactic space.
The Star Wars Expanded Universe adds enough of these that it seems kilometer-plus is fairly standard for heavy capital ships in the Galaxy Far, Far Away, particularly in the Imperial and post-Imperial eras. These are just some of the biggest examples.
"Super star destroyer" is basically a catchall term for ships following the star destroyer design philosophy and larger than about three kilometers. The Empire built many of these, but most were one-offs and only the Executor-class went into (relatively) wide production with around a dozen known.
By the New Jedi Order series the New Republic saw the need to have a counter for super star destroyers and the like, and the Mon Calamari supplied the 17 km Viscount-class battleship. For their part the Yuuzhan Vong had their koros-stronha or "worldships", 10 km in diameter and primarily colony ships but quite capable of defending themselves against GFFA capital ships. They also had a couple different types of kor chokknote "grand cruiser" of similar size that served much the same command-and-control/heavy battleship role as super star destroyers.
By the Legacy comics, ships of SSD scale are seen as Awesome, but Impractical (without a galaxy-sized tax base they cost too much to operate) and no longer in use. The Fel Empire's warship of choice is the Pellaeon-class star destroyer, which is about the same length as an Imperial-class but more massive.
The two miles long Imperial battlecruiser from Foundation.
In John Ringo's Troy Rising series, Troy, Thermoplae, and Malta are mobile battle stations 9 to 13 km wide, each using rapid-fire 50-megaton nukes to power their Orion Drives and carrying an entire fleet of cruisers, fighters, and millions of missiles. They make the Rangora (the enemy aliens) Assault Vectors (which are pretty impressive battleships in their own right) look puny.
In Chindi, part of Jack McDevitt's Priscilla Hutchins series, the alien ship known as the chindi is over 16 km long, and apparently uninhabited, but still intent on its unknown mission.
Ships in the Honor Harrington series are usually denoted by their tonnage rather than their length, but one book had a diagram that showed their main warships as being 2 kilometers in length for a battleship to 3 km for a Superdreadnought. Then David Weber apparently realized that at that scale they were far too light and he re-scaled them so they top out at just over 1500 m for an SD and 1200 m for a battleship.
In Reynolds' standalone novel, House of Suns, the ships are even longer - the protagonist's ship is 50 kilometers long. Most of the mass and length is taken up by the slower-than-light propulsion systems, but the ship still has room for an 8 mile long cargo hold which carries smaller interstellar ships inside. The novel also briefly shows moon-sized ships, though their mass makes them agonizingly slow, especially for the already slow, non-FTL interstellar travel.
Ships with a length/diameter of 1500 meters or more are nothing special in the setting of Perry Rhodan. There are many examples of giant ships or mobile battle stations, for instance the cosmocratic spore ships, spheres with a size of 1126 kilometers.
Some of the starships in Iain M. Banks' The Culture novels are quite large — a GSV (General Systems Vehicle) can be 25 to 200 kilometers long, with millions of passengers and enough on-board manufacturing capacity to match a planet in production. Consider Phlebas shows the megaships of Vavatch, which are massive oceangoing city ships that are at least 4 kilometers long, and are so massive that it takes years for them to accelerate to their max speed. The megaships use enormous cruise ships as tenders.
One scene taking place on a GSV contains a throwaway observation by one of the characters of a megaship being neatly tucked away for storage in one of the GSV's smaller storage bays. Yeah.
In the Star Carrier series by Ian Douglas the eponymous vessel America is a kilometer long and not very wide. There are also several Turusch vessels built from hollowed-out asteroids that are several times larger. H'rulka vessels are an interesting version: they initially look 20 klicks long but are actually several single-crewperson ships docked together that are individually several klicks in length (the creature flying it is a 200-meter colony organism).
According to the website of the author of The History of the Galaxy series, ships in that 'verse tend to be fairly large, almost absurdly so, especially since the stated crew complement is very small (although that could be justified by mass-produced AI modules that can be used to replace crew). The largest warships of the Standard Sci-Fi Fleet, flag cruisers, are 7 kilometers (almost 4.5 miles) long... with a standard crew of 150. Slightly justified by a fifth of the ship being taken up by its Anti MatterWave Motion Gun (it's almost never used and really only serves as a deterrent). The first human extrasolar colony ship, the Alpha, was even larger. However, the Alpha was built before the discovery of Hypersphere and was supposed to have been propelled by three massive fusion engines. A large chunk of the ship was devoted to the engines and their hydrogen fuel. Even then, half the colonists/crew were supposed to be kept in stasis at any given time, with shifts rotating every couple years.
In Sergey Lukyanenko's Genome, the main character notices a Taii ship, a Moon-sized sphere, patroling their ancient borders (which now belong to younger races). Subverted in that this ship is a relic of bygone times. The Taii ship is escorted by a tiny human destroyer that can incinerate the giant sphere with a single volley.
In the Ender’s Game prequel Earth Unaware the Formic mothership is at least a kilometer in diameter.
Of course in most series it has an organic crew complement of "two", plus several robots, the ship's computer, and a Virtual Ghost, none of whom use anywhere near all of the ship. So the inconsistencies might be excusable.
Star Trek doesn't normally go in for this, but there are a few examples.
The Deep Space Nine episode "Valiant" provides a Jem'Hadar battleship described as twice as the size of a Galaxy-class starship. A quick calculation based on the DS9 Technical Manual results in a ship just under 1.3 kilometers long. Another Jem'Hadar capital ship that is similar but much larger is seen during the Battle of Cardassia in "What You Leave Behind".
One episode of Star Trek: Enterprise has Captain Archer being taken through time and brought on board the USS Enterprise-J. Due to time constraints with making the episode, the designer of the ship admits that they only had a couple of weeks to flesh out a concept, which is only very briefly shown in a fuzzy holographic computer display in the episode itself. The ship was never shown from the outside in the episode, so supplementary material filled in for that. The Enterprise-J was built as a generational ship, capable of folding space to instantly travel to other galaxies. It is so large that its turbolifts are replaced with site to site transporters, and it features massive parks and even an entire university on board. Its saucer is roughly 26 miles in diameter.
The Romulan D'deridex class warbird is slightly over a kilometer in length. Most of it is empty space though.
Even Starfleet is close to the trope, with the Enterprise-E being close to half a mile in length.
The spinoff Crusade had the Excalibur, which was 3000 meters in length, and had an internal rail line (similar to that on the Babylon 5 space station) running along its length for transportation within the ship.
Also, the Babylon 4 station, which preceded Babylon 5, had a propulsion system (unlike its successor), so could serve as a mobile base-ship and carrier that was 6 miles in length (Babylon 5, meanwhile, was 5 miles in length).
There were also the Explore-class vessels. Their task was to travel to unexplored systems, identify those worth permanently occupying, and deploy jumpgates. They were several miles long, and so broad the pylons of the Babylon 5 jumpgate had to be spread wider than normal to create a jump point large enough for them.
Minbari Sharlin-class war cruisers are about the same size as a star destroyer, though that's in height, not length (they're taller than they are long).
The largest ships to date are Vorlon Eclipse-class planet killers. Estimates put them at 26-36 miles in diameter (they're circular). And they're definitely visible from planetary surface.
Pretty much every major Earthforce warship measures around a kilometer, with the Omega-class destroyer breaking the mile barrier at 1700m, and it's successor, the Warlock-class making over 2000.
The Ori warships in Stargate SG-1 were established by ancillary materials as 1.1 kilometers long.
Wraith hive ships in Stargate Atlantis were described as roughly thirteen times larger than an SGC Daedalus-class battlecruiser. This gives a ship around 2.9 kilometers long. The super-hive that appears on the series finale is much larger, making it the largest known starship in the setting. Meanwhile, Ancient city-ships such as Atlantis are about 3 kilometers in diameter.
In the show Lexx, the Lexx itself is an insect-like ship that is 10 kilometers in length.
In the Imperial Navy alone, escort frigates and destroyers are 750 meters to 2 kilometers long, while battleships are up to 8 km, and some go even bigger than that. Rogal Dorn's Phalanx is what you get when you take an asteroid and stuff some weapons and engines on it; it's the size of a small moon.
Eldar craftworlds are even bigger. Because it's 40k, different books say completely different things; some say they're the size of planets, others say they're "only" a few dozen kilometres long.
And even ordinary JumpShips tend to be over a kilometer in diameter, though that's just the "sail" used to gather solar energy, the ship proper is usually around half a kilometer long.
In Mass Effect, Reaper capital ships like Sovereign and Harbinger are around two kilometers long and so massive that their kinetic barriers are severely weakened while they're landed in Earthlike gravity (due to the mass effect field strength needed to reduce their mass to the point where they don't crumple under their own weight). Citadel dreadnoughts vary by race, but average one kilometer. Ships of that size are justified by the setting's reliance on kinetic weapons: a longer ship means the spinal mass accelerator can be longer, meaning it can fire its slugs with greater force. For these reasons, massive dreadnoughts are used like self-propelled artillery pieces, remaining at the rear line of a fleet battle and exploiting their greater muzzle velocity to take down targets while smaller, lighter ships either screen their flanks or maneuver in around the enemy's flank. (At least, that was the idea. Nobody apparently told the VFX artists, who in Mass Effect 3 showed Citadel dreadnoughts engaging Reapers well within visual range.)
The X-Universe series doesn't explicitly give lengths for its ships. However, in 2009, a forum member in this thread scaled their in-game models and determined that the largest destroyers average 3 kilometers, with the ATF Valhalla being the outlier at 5. The Valhalla's sheer size makes it wildly impractical to use normally, because its engine nacelles will impact the rim of a jump gate upon exit, causing it to get stuck inside the event horizon. This is fixed by the expansion pack X3: Albion Prelude, but it still has size-related problems with its turret firing arcs. Our page picture is the more manageably sized Teladi Phoenix, which is approximately 4 kilometers long. The games also features some truly enormous space stations, such as the Terran Military Outpost, a station so large that the entirety of Rhode Island could fit within its bulk, or the Torus Aeternal, a station large enough to wrap around the Earth.
Aurora Carriers are 1.2 km long, compared to the Federation and Rebel Carriers which are only 500 m long. Note they carry many more fighters than their counterparts and the Aurorans love their railguns.
Polaris Ravens are 1.2 km long and about 1 km wide making them the biggest playable ship in the game. Strangely, they have a crew of less than a hundred.
Deimos-Class Colony Ships are approximately 3×3×3 km, as the hypergates they traveled through had to be reconfigured to form wormholes with a diameter of 5 km.
Titan class ships in EVE Online can be as long as 18 kilometers. Other ships, while still being a fraction of the Titans' size, are still enormously long.
The Mothership from Homeworld, which the fluff says is nearly 15 kilometers "tall"note relative to the direction its main engines face, see its entry's page picture for details. It has a major case of Units Not to Scale in-game, however, but still measures a good three in-universe kilometers... Or rather "kloms", whose relationship to an Earth kilometer is never mentioned.
The trope becomes evident as early as the final level of Halo: Combat Evolved when one of the waypoints during your escape from the Pillar of Autumn is over a kilometer away. The Autumn's official length is 1.17 km. The colony ship Spirit of Fire from Halo Wars is 3.5 km long, and the experimental vessel Infinity is 5.7 km long, capable of carrying an army and ten internally docked frigates, and was meant to be Plan B if Earth was destroyed. Human ships' main weapons are magnetic accelerator cannons (read: railguns), so the trope makes sense for similar reasons as in Mass Effect, above.
Covenant capital ships are even bigger, with the shining example being the CSO-class supercarrier that appeared during the Battle of Reach (Halo: Reach), an utter behemoth at just under 29 kilometers long.
And then there's the Forerunner Fortress-class vessels, which are 50 kilometers long. Later, in Forerunner-Flood war, Fortress-class reach new height with 100 km long vessels, pushing them into Planet Spaceship territory.
Common in FreeSpace, in which any Destroyer-class or above ship is going to be at least 1.5 kilometers long. The Colossus and Sathanas measure in at nearly six kilometers long as the largest ships in the game. Fan-made expansions sometimes feature even larger ships (the current record-holder being the Gargant, at over 60 km).
In Ground Control, the CSS Astrid is a massive warshipnote a rough fan estimate puts it at almost exactly one mile. that contains enough men and materiel to lay siege to an entire planet by itself, and in the sequel, evacuate the majority of the populace of Morningstar Prime.
In Star Trek Online, the Borg Unimatrix 0047 Command Ships, hexagonal juggernauts roughly 20 kilometers in length. They're so big they don't even maneuver: they just sit there and spam various attacks at you. On a much smaller scale, Klingon Vo'Quv-class carriers sit at around a klick in length and can deploy Birds-of-Prey instead of conventional fighters.
Sunrider: The PACT super dreadnought, Legion, which is three kilometers long.
The Ultimate Starship Size Comparison Chart lines up notable ships and space stations from several different science fiction franchises (along with a few real-life ships for the sake of argument) side-by-side to compare their length. They're all dwarfed by the aforementioned Executor-class star dreadnaught from Star Wars.
There are currently no Real Life examples of a mile long ship, but that doesn't mean that there aren't currently some impressively large ships. In the US, football fields are often used as a stock unit of measurement of measurement for such ships, with a particular vessel being described as 'x' football fields in length.
For reference, the biggest ship ever built was the 650,000-ton supertanker Seawise Giant. She was 1,500 feet long (just over a quarter mile, or 0.46 km). Meanwhile, the biggest military capital ships currently in use are the Nimitz-class supercarriers, which are a comparatively paltry 333m and 100,000 tons.
Seawise Giant possessed the greatest deadweight tonnage ever recorded. Fully laden, her displacement was 657,019 tonnes (646,642 long tons; 724,239 short tons), the heaviest ship of any kind, and with a draft of 24.6 m (81 ft), she was incapable of navigating the English Channel, the Suez Canal, the Strait of Malacca or the Panama Canal, and incapable of entering the Baltic Sea. Overall, she was generally considered the largest ship ever built, as well as the largest self-propelled human-made object ever built. Her design approached the practical limit of ship design - larger ships than Seawise Giant are physically possible to be built, but they would be too impractical to use.
There has been an apparently serious proposal made to build a mile-long floating city ship. Buoyancy would be from a system of interlocking modules that would each have their own buoyancy tanks and propulsion units. This should allow a ship much larger than conventional construction methods would allow. (A normal ship in rough seas can flex to a degree quite alarming to those not familiar with modern vessels. The flexing limits the practical size of conventional vessels as well,)
JPL scientist Robert Frisbee wrote a paper called How to Build an Antimatter Rocket for Interstellar Missions. His interstellar antimatter rocket involves a ship not merely a mile long, but 435 miles long (700 km). This is necessitated by the need for lots and lots of room both for storing antimatter and for a rocket fueled by it.
Our current largest spaceship (actually a space station; its rockets are only to help keep it in orbit) is around a tenth of a kilometer wide, and around 1/14 kilometer long (according to The Other Wiki). This sounds more impressive than it is, due to the space station's shape.