Tiny

2 1/2 by 2 1/2 ft.

Imp, sprite

Small

5 by 5 ft.

Giant rat, goblin

Medium

5 by 5 ft.

Orc, werewolf

Large

10 by 10 ft.

Hippogriff, ogre

Huge

15 by 15 ft.

Fire giant, treant

Gargantuan

20 by 20 ft. or larger

Kraken, purple worm