how do i spawn the biggest thing in game

[Kat]

i want to see just how big the creatures are. whats the command to spawn it?

[Tim Timsen]

Spawn Cuddlefish

[TheNextHorizon]

spawn rockgrub
just kidding, i think the biggest are the ghost leviathans or reefbacks, or the sea emperor and sea dragon

http://steamcommunity.com/sharedfiles/filedetails/?id=1286859932
the tiny green light is the rockgrub

[CrazyGoblin]

http://subnautica.wikia.com/wiki/Console

Should provide the commands and instructions you're looking for. Technically speaking, several of the game's enemies (particularly the rather large ones) are something of a "spoiler" in that the game intends for you to experience these unknown beasts firsthand, and quiver in fear of their strangeness.

If you don't care about that, the largest wild creature would be the seadragon (entered as one word, when used in conjunction with the associated spawn command).