Consult the Size Categories Table in the Monster Manual (p. 6) or the Player's Handbook (p. 191).
The Monster Manual defines creature size as:
how much space a creature of a particular size controls in combat.
Size | Space |
---|---|
Tiny | 2.5 x× 2.5 ft |
Small | 5 x× 5 ft |
Medium | 5 x× 5 ft |
Large | 10 x× 10 ft |
Huge | 15 x× 15 ft |
Gargantuan | 20 x× 20 ft or largeror larger |
A creature's Size category does not directly define its ohysicalphysical size, rather it defines the area that it controls onin combat. A horse is size Large, but obviously horses are not 10 feet long by 10 feet wide. And on the other end of that, a gargantuanGargantuan creature has no upper bound to how largebig it can be - it may control far more than a 20 feet by 20 feet-ft.-by-20-ft. area in combat.
The Size section of the basic rules contains this info as well.