Size of creatures is a key mechanic which, importantly, determines how many creatures can attack a creature in a single round. These rules are found in DMG 69, and are detailed more on the Targeting page.

Humans, elves, and half-orcs can be all considered M-size, since they typically fall in the 5'-7' range mentioned on MM 6. Dwarves, gnomes, and halflings are S-size, since they typically fall under 5' in height.