Sentient aliens can come in many varieties, from human lookalikes
to Rubber-Forehead Aliens
to aliens that resemble certain Terrestrial animals
to Starfish Aliens
. But a great majority of depicted sentient aliens, even Insectoid Aliens
, are depicted as being similar in size and stature to humans. Sentient aliens that are much larger than humans are usually Eldritch Abominations
of some sort, and sentient aliens that are much smaller than humans are very rare.
On Earth, humans may or may not be the only fully sentient
species (dolphins and chimps at least come close), but there are many smaller animals that have adopted an evolutionary strategy that uses intelligence--squirrels, raccoons, crows, wolves/dogs among them--and might have conceivably evolved into the dominant species were it not for man (through his line of forebears) getting there first. Simple brain size isn't the factor, but there is probably some correlation to brain size in proportion to body mass. But even basic biological principles (such as how neural systems function or are structured) might vary significantly from one life-bearing planet to the next. So conceivably, sentient aliens needn't be of a certain size range at all, so long as whatever size they are worked within their native ecosystem well enough that they could evolve sentience.
This trope seems so common that we could probably just look for inversions. [I can only think of three right now, although I may be overlooking some.]
- In the novel The Hitchhikers Guide to the Galaxy, a formidable and deadly fleet is described in a side-story as threatening Earth, but when it arrives it is consumed by "a small dog"--implying of course that the aliens are nowhere near human size, probably microscopic.
- Earth's mice are also revealed to be sentient aliens from another dimension.
- Rygel in Farscape is fairly diminutive at around two feet tall (with the other main alien characters near human size, or a bit larger like Pilot or the Scarrans). Due to groundbreaking use of puppetry in the Space Opera genre, diminutive but fully animated sentient aliens like Rygel, along with a variety of Starfish Aliens, were possible without CGI.
- Then there's the Living Ship Moya of the Leviathan race, who seems fairly sentient and is hundreds of meters long.
- Yoda in the Star Wars franchise stood about two feet tall.
- And Jabba the Hutt was a massive blob of an alien that probably weighed several tons.
- Another Star Wars aversion is Col. Meebur Gascon from The Clone Wars. He looks more or less like a classic folklore Martian: a little (30-40 cm tall) green man with eyes on stalks.
- Animorphs has a lot of scale variations: The Helmacrons are microscopic (relative to them, a fly is smaller that a molecule), the Hork-Bajir are well over seven feet tall, and the Yeerks are slugs the size of rats.
- Megas XLR: I can't remember if the species had a name or not, but one episode has a human-sized bounty hunter employing Godzilla-sized rhinoceros-like creatures. At the end it's revealed the huge aliens are sentient and quite civilized.
- Warhammer 40K: except for the Tau and the Demiurg/Squats, humans are a rather short sentient species in the galaxy (Orks, Kroots and the smarter Tyranid forms start at seven feet tall while Eldar are Space Elves and therefore taller).
- On Ben 10, Ben's most intelligent alien form is Gray Matter, a frog-like humanoid about one foot tall.
- Named alien characters in the Men in Black films range from the size of an action figure to the size of a subway train. Some scenes suggest that other galaxies or universes exist that are thousands of orders of magnitude larger or smaller, with inhabitants in proportion to their scale.