Size, Space, and Reach

Creatures and objects of different sizes occupy different amounts of space. The sizes and the spaces they each take up on a grid are listed in the Size and Reach table (see below). The table also lists the typical reach for creatures of each size, for both tall creatures (most bipeds) and long creatures (most quadrupeds). See Range and Reach for more about reach.
The Space entry lists how many feet on a side a creature's space is, so a Large creature fills a 10-foot-by-10-foot space, or 4 squares on the grid. (If you need to measure in three dimensions, their space is also 10 feet high.) Sometimes part of a creature extends beyond its space, such as if a sky fisher is grabbing you with its lasso filaments. In that case, the GM will usually allow attacking the extended portion, even if you can't reach the main creature.

A Small or larger creature or object takes up at least 1 square on a grid, and creatures of these sizes can't usually share spaces except in situations like a character riding a mount.
Multiple Tiny creatures can occupy the same square. At least four can fit in a single square, though the GM might determine that even more can fit. Tiny creatures can share a space occupied by a larger creature as well. If a Tiny creature's reach is 0 feet, it must share a space with a creature in order to attack it.