Seldom has 'starship' been so literal... or insufficient.
There's big spaceships, there's gigantic spaceships, and then there's these: a space ship (or station) that's as big as a small planet or large moon. In space, you have no external gravity to contend with, so it's possible to build some truly humongous structures like this. Alternatively, you may just want to strap some engines onto an actual planet or moon and set off, although this may require Sufficiently Advanced Technology.
Often used for Rule of Cool, in which case it will overlap with Cool Starship, the Planet Spaceship is a great way to intimidate your foe, or to pack your entire population off when your home world is in trouble. May evoke a "That's No Moon" reaction when it first appears.
Technically, actual planets and moons are spacecraft in the sense that they are in orbit around something. Buckminster Fuller made this analogy when he referred to "Space Ship Earth." But to qualify for this trope, the super-colossal spacecraft must have some means of propulsion, or at the very least orbital correction.
Subtrope of Mile-Long Ship. May be used as a Big Dumb Object, The Battlestar, or a Generation Ship. Commonly, though not exclusively, found in Space Opera. Compare Dyson Sphere and Ring World Planet. See also: Unnecessarily Large Vessel, Unnecessarily Large Interior and Awesome, but Impractical.
Mogo, the Sentient Planet is a Green Lantern, and effectively becomes this as he can travel through hyperspace.
Film - Animated
Titan, for which Titan A.E. is named, is a very large, spherical spaceship. When activated, it converts energy into mass and turns into an actual planet.
Film - Live Action
In Star Wars, the Death Star is a moon-sized superweapon which is the original source of the That's No Moon trope. For specificity's sake, the first Death Star was established as 160 kilometers in diameter, and the second would've been 900 kilometers or twice as big as the first one (there are contradictory reports) if it hadn't been blown up first.
In the Ted Reynolds short story, "Ker-Plop", we get a reminder of the difference between size and volume, when humans return from the Magellanic Clouds in a ship that is ten thousand kilometers across (about 1/10th the diameter of Ceres). The agent sent to investigate realizes that because the entire volume is inhabited, rather than just the surface, this one ship contains more people than his entire galactic federation. The decks are arranged concentrically, like an onion, and the first kilometer alone has nearly 400 decks, each with nearly the same surface area as the entire ship!
In E. E. “Doc” Smith's Skylark Series, Drs. Seaton and Crane build a 1000km diameter spaceship called the Skylark of Valeron. Its size was needed to house the sensors required to travel at its full velocity. Later, their rival Dr. DuQuesne builds an even bigger ship.
In Larry Niven's Known Space series, the Pierson's Puppeteers are a race of cowards, and the only Puppeteers humans have met are insane— because no sane Puppeteer would trust his life to something so fragile as a spaceship. So when they discover a massive wave of radiation approaching their home system, they leave it behind... but take their planets with them. They put them in orbit around each other (see Klemperer Rosette on The Other Wiki) and accelerated them to just under lightspeed, heading out of the galaxy. Because humans have faster than light starships (which Puppeteers are too sane to use), they expect humans to greet them at their destination, the Clouds of Magellan.
The planet Gor itself can be considered an example, since the Priest-Kings (the Physical Gods of the planet) moved it to its current location 5 million years ago.
The Kurii live in "Steel Worlds" in the asteroid belt; from there they plot their plans to destroy the Priest-Kings and take over Gor & Earth for themselves. The Steel Worlds have aritifical weather & daytime/nighttime and rotate to simulate gravity, with beings living on the inside circumferance of the ships. They used to have a planet of their own but they destroyed it making war with each other.
David Weber's Empire from the Ashes series has Dahak and all the other Imperial Planetoids. Dahak has spent the last 50,000 years pretending to be Earth's Moon... and it's the smallest of them. They come equipped with hundred kilometer thick armor and carry 80,000 ton battleships as parasite craft.
Varley also uses this trope in his short story "The Funhouse Effect": a large comet in a close solar orbit is partially hollowed out, has engines mounted on it and is being used for exclusive pleasure cruises 'round the Sun.
In The City and the Stars by Arthur C. Clarke, it is revealed at the end that most of the humanity left the Galaxy to explore the universe... in a star cluster made into a fleet.
At the end of Halo: The Cole Protocol, the inhabitants of The Rubble, survivors of the Covenant invasion of their system, convert a large asteroid into an evacuation ship to get everyone safely to UNSC space.
In Remnants, the humans who awaken when the ship reaches a destination initially think they've landed on a bizarre alien world. They soon discover it's a massive spaceship controlled by a sentient AI that brought them aboard as a curiosity.
In Robert Reed's Great Ship series, the Great Ship itself is an ancient abandoned artifact which was claimed and settled by mankind's descendents. It is larger than Jupiter. The interior of the ship has hundreds of thousands of enormous caverns which are inhabited by the passengers. The ship has fourteen gimbaled fusion engines on one end, each of which is large enough to fit several dozen Earth-sized planets within their nozzles, which are fed from dozens of hydrogen fuel tanks whose individual volume exceeds that of all the Earth's oceans combined by several orders of magnitude. In The Well of Stars, a planet weapon is shown - the Sword of Creation - a planet-wide hyperfiber ring which has microscopic black holes tethered to its rim, which is then spun up and repeatedly launched at planets to expose a path to its core for mining.
Cordwainer Smith gives us The Golden Ship - a sphere 90 million miles in length (just 2 million miles short of an AU). That's approximately 100 times larger than Earth's sun! Extremely fast, it could move in seconds. However, the Golden Ship is really a decoy, being mostly hollow and consisting of foam and wires. It has no weapons of its own, and must rely on its speed to survive.
The largest vessels in the Perry Rhodan series fall under this trope. For instance the cosmocratic spore ships, spheres with a size of 1126 kilometers.
Terre en fuite (Fleeing Earth) is a novel by François Bordes where Earth and Venus are turned into giant spaceships in order to allow humanity to escape the Sun going nova. Initially, the plan is to wait out the explosion behing Jupiter and come back. However, scientist then determine that the Sun will not return to its former state, so they have to move the planets to another system... and then again when the first one turns out to have a Lost Colony of humans.
In The Witches of Karres, the so-called witches use their "Sheewash drive" to move the entire planet of Karres around when they need to.
The larger models of Culture General Systems Vehicle from Iain Banks' The Culture fall here. The things can be anywhere from 25 to 200 kilometers in size, and carry millions of passengers and can match planets for industrial production. One scene shows four kilometer oceangoing ships being packed away for storage inside a GSV.
In the Star Carrier novels the Turusch build their larger warships by hollowing out large asteroids and adding engines. An extreme example of this is the Regrets of Parting, made from a dwarf planet at least 900 kilometers in diameter and massing about 900 quadrillion tonnes.
The Polish sci-fi novel for teenagers Ci z dziesiątego tysiąca (Ones from the Tenth Thousand) by Jerzy Broszkiewicz mostly takes place on a "mechanoplanet": a large, intelligent spherical spaceship with artificial environment.
In Andromeda the Magog come from worlds (yes, that's plural—twenty of them) locked together in some kind of structure. The whole thing is mobile, FTL capable, and even contains an artificial sun. Oh, and it can survive a hit from a Nova Bomb, the largest weapon in the Commonwealth arsenal, normally capable of nuking entire solar systems.
In the Star Trek: The Original Series ep. "For the World is Hollow and I Have Touched the Sky" a group of aliens have been sequestered inside a large interstellar asteroid for so long that they have forgotten that they are actually inside one.
Doctor Who has had one or two in its 50-year history. First, the Daleks attempted it in "The Dalek Invasion of Earth," but failed. The second, and somewhat more successful of these, was Zanak from "The Pirate Planet," a hollowed-out planet with massive transmat engines...it could materialize around another planet and capture everything from it, leaving behind a tiny husk, which its captain could collect as a trophy.
The TARDIS itself probably is this. Just not on the outside.
Eldar craftworlds are described as "planetoid-sized".
The Necron World Engine, unsurprisingly, is a Necron ship the size of a planet (because they slapped an engine on a planet). An entire space marine chapter sacrificed itself to destroy it.
Also, the Phalanx is the moon-sized fortress-monastery-spaceship of the Imperial Fists.
World Of Synnabar takes place in a spaceship made from a hollowed out Mars.
In Table Top Game/Pathfinder, Apostae, the tenth planet from Golarion's sun, called "The Messenger", is one of these. It's a Generation Ship, with it's crew/cargo trapped inside, horribly mutated and deformed, having long sense lost control of the ship and memory of their mission, if they ever had it to begin with.
In BIONICLE, Mata Nui was designed to observe other universes while having beings inside him functioning as Nano Machines in a sense. His Humongous Mecha body is roughly forty million feet (7,575 mi or 12,192 km) tall and had a camouflage system that created an island on his face. Mata Nui was able to control specific things like gravity and light within his body.
In Marathon, the eponymous ship was originally Mars' moon Deimos. This is less impressive than it sounds given that Deimos is about six kilometers long... fairly big by human standards, but nowhere near the size of a planet.
The Dammerung in Xenosaga is described in the accompanying Perfect Works manual as an artificial planet the size of Lebanon. It's the headquarters for Fiction 500 company Vector, and contains at least one megalopolis in an area large enough to have a real weather system.
In the videogame adaptation of Futurama, Mom's plot involves transforming the Earth into a giant spaceship.
In Star Ruler, the players can make ships anywhere from the size of a coke can to ships larger than the galaxy. At around ship scale 1000, ships start to become larger than the planets that build them. Additionally, it's possible to turn entire planets into ships by researching the Planetary Thruster. Building a bunch of planetary thrusters, some shield generators, and planetary laser cannons essentially turns the planet into a ridiculously durable, but relatively poorly armed ship with the capacity to store thousands of smaller ships, or buildmore ships on the way to its destination by importing goods from the galactic bank.
Sonic the Hedgehog has The Death Egg, a parody of the Death Star. The intro of Sonic & Knuckles illustrates just how big it is—the Eggman face fills the top of a volcano (Lava Reef, to be exact).
The X-Universe series' backstory provides an extreme example. Billions of years ago the Ancients fought a several-million-year-long war against a Kardashev Type V+ race called the Outsiders who are believed to have invaded from another universe. They entered ours in the form of several ships that were each the size of a solar system. (Yeah, Sufficiently Advanced Alien doesn't even begin to cover this.)
The first two games in the Might and Magic series has an implicit case: the worlds they take place on are indicated to be just two out of several carried by an Ancient automated vessel. Both CRON and VARN (the worlds in question) are decently-sized themselves, giving some inkling of the minimal size of the vessel. The third game does not actually take place on this ship, but the islands on planet it does take place on were once smaller such worlds on the ship that were placed on the planet by the Ancients as colonies. On a more minimal scale, the villain of World of Xeen (the combined IV/V) planned to use XEEN as this (the problem is that while it would indeed be entirely possible to do this, it'd kill the natives, at least with the villain's implementation).
The Voth fortress-ship in the Star Trek Online PVE raid "The Breach". Starfleet's Odyssey-class has been scaled as roughly a kilometer in length, and this thing probably dwarfs a hundred Oddys nose-to-stern. Oh, and it carries multiple Citadel-class dreadnoughts within it like fighters, each of them roughly 10 km long.
In Invader Zim, the original inhabitants of Mars drove themselves to extinction converting their planet into a giant spaceship.... "Because it's cool". a similar thing happened to Mercury, which leads to awesome dogfight.
Shadow Raiders feature a handful of planets with engines designed by an unknown precursor race.