Consider a monk with imp unarmed strike. Medium sized he has a reach of 5 ft as normal. If grappled he can fight effectively using imp unarmed strike.
Then he gulps down a potion of enlarge person, becomes large and now has a reach of 10 ft. If grappled he can still fight effectively using imp unarmed strike.
My question is, does the enlarged monk with 10ft reach still threaten at 5ft range? (A fighter using a 10 ft reach weapon wouldn't, but I wonder if a monk could simply strike with elbows, knees and headbutts at that range and still threaten?)
Lady-J is correct. Since Unarmed Strikes aren't reach weapons, they would threaten at 5' as well as 10'.
Large, Huge, Gargantuan, and Colossal Creatures: Very large creatures take up more than 1 square.
Creatures that take up more than 1 square typically have a natural reach of 10 feet or more, meaning that they can reach targets even if they aren't in adjacent squares.
Unlike when someone uses a reach weapon, a creature with greater than normal natural reach (more than 5 feet) still threatens squares adjacent to it. A creature with greater than normal natural reach usually gets an attack of opportunity against you if you approach it, because you must enter and move within the range of its reach before you can attack it. This attack of opportunity is not provoked if you take a 5-foot step.
Large or larger creatures using reach weapons can strike up to double their natural reach but can't strike at their natural reach or less.
Nice, thanks. That seems pretty straightforward then :)