Keep it single-faceted: momentum isn't affected, meaning he'd swap momentum and place with an object. As for the volume part, here: suppose your character is a 1 m - wide cube (yes, not realistic, but beginner physics rarely involve something other than spherical cows on infinite frictionless planes). A percentage limit means that he couldn't swap with something either X% larger or Y% smaller. Personally, I suggest being really severe on the Y, less for balance and more so he doesn't end up in a space he can't fit in, which may or may not kill him or her instantly. Apart from that, it would mean that Guy can swap with something (let's say the percentage limit is 10% for the sake of example) larger than 1.1 cubic metres.
Do keep in mind that whatever the semblance, it needs to be balanced for his/her age and past.