Tiny: Four individual Tiny creatures can fit in a square, and a Tiny creature can enter a larger creature’s space and end its turn there. Having a reach of 0 means a Tiny creature cannot make melee attacks against targets outside its own space, unless otherwise noted. Examples: mundane animals such as cats and crows.
Small: Small creatures occupy the same space as Medium creatures, but they face some restrictions in the weapons they can wield (see “Weapons and Size"). Examples: gnomes, goblins, halflings.
Medium: This is the size of a typical human and serves as a standard in the game. This category has no special rules. Examples: humans, dwarves, elves, orcs.
Large, Huge, and Gargantuan: Creatures larger than Medium take up more than 1 square. For instance, an ogre takes up a space that is 2 squares by 2 squares. Most Large and larger creatures also have a reach greater than 1, which means they can make melee attacks against creatures that aren’t adjacent to them. A creature’s basic body shape usually determines its reach; a Large ogre has a reach of 2, but a Large horse has a reach of 1. Examples: dragons, giants, trolls.
Published in Rules Compendium, page(s) 201.