See also: Character Combat
A creature's Reach determines the distance it can Reach when making a Melee Attack. A creature Threatens all squares within its Reach. Small and Medium characters have a Reach of 1 square, which means they can make Melee Attacks only against targets in adjacent squares. Larger creatures tend to have a greater Reach and, respectively, a bigger Threatened Area.
Bigger Creatures
A creature with greater-than-normal Reach (More than 1 square) can still attack opponents directly next to it. A creature with greater than normal Reach usually receives an Attack of Opportunity against an opponent when the opponent approaches it, because the opponent must enter and move within its Threatened Area before making a Melee Attack.
Smaller Creatures
A Fine, Diminutive, or Tiny creature must be in your space to Attack you; moving into your square provokes an Attack of Opportunity. You can Attack into your own space if you need to with a Melee Attack (But not a Ranged Attack), so you can Attack very small opponents normally.