Re: How big in diameter for wheels?
Not sure what the XM's are like guard wise but I do have a vague idea. I'm guestimating you could so more than a 16" rim. Don't worry about if it clears the front guards as most will. There is more room in the front inner guard.
It's the rears where you come unstuck.
I found this out with my XB when from 14's to 16's. Any more than a 16" wheel means guard lipping/inner guard modifications/weird wheel offsets to clear suspension. I've got almost no clearance.
XB Fairmont (street car) email@example.com.