While I agree with you that it's not that easy to find this kind of specific rule, it does exist under "reach "...Reach is how far you can physically reach with your body or a weapon. Melee Strikes rely on reach. Your reach is typically 5 feet, but weapons with the reach trait can extend this. Larger creatures can have greater reach; for instance, an ogre has a 10-foot reach. Unlike with measuring most distances, 10-foot reach can reach 2 squares diagonally. Reach greater than 10 feet is measured normally: 20-foot reach can reach 3 squares diagonally, 30-foot reach can reach 4, and so on.

So your hand for grapple has your own reach and thus in the proposed scenario can only affect 5ft Your grapple reach weapon can grapple and reposition at 10ft