OK, so, another thread got me thinking. If I have a small fighter that is hit with reduce person, he becomes tiny, with a 2.5' square base and 0' reach. What if he uses a reach weapon, would he then have a 5' reach?
Don't know what the RAW is, but that's how I've always ran it.
Reach Weapons: A reach weapon is a melee weapon that allows its wielder to strike at targets that aren't adjacent to him. Most reach weapons double the wielder's natural reach, meaning that a typical Small or Medium wielder of such a weapon can attack a creature 10 feet away, but not a creature in an adjacent square. A typical Large character wielding a reach weapon of the appropriate size can attack a creature 15 or 20 feet away, but not adjacent creatures or creatures up to 10 feet away.
So, as written, he would still have a reach of 0'.
To be clear though, I would probably play it as 5' too. Makes dealing with Tiny creatures way, way simpler.
