Aliens are depicted in many forms, though the original is a little grey or green martian men from Mars, like in the spoof film Mars Attacks or Independance Day. Usually they have large craniums or protruding brains, which is supposed to make them super intelligent and give them extra powers which humans don't have, like telepathy, telekinesis and sometimes advanced healing.
In the movie Paul, most of these things are taken into account, including the extra abiliy of invisable camouflage. They have a maximum height of usually four feet, but can range from any size and body mass, though they are more depicted sometimes so thin, i'm surprised they can support their huge heads.
With the introduction of Doctor Who now being in regular circulation, there are many more alien monsters which, even though suspiciously human looking, are incredibly varied and mosterous looking. Most have advanced cybernetics and robots can usually be alien as well.
But as well as intelligent aliens, like the Predator or the Face of Boe, there are also many primal aliens, like The Alien or the Slithers from well named movies Alien and Slither. But being so primative, they have better natural defences like poisons, tails and speed. But the range of aliens is almost limitless, so you really can create your own; whether their humanoid ones like in Prometheus, or giant gold sucking bug ones, like in Cowboys vs. Aliens.
