you could probably use a 2.25 clamp. i would imagine a 2.50 would be too big but i could be wrong
one trick i use when clamping on mufflers/exhaust pipe is clamp it down real tight then loosen it back up and spin the clamp around 180 degrees so its opposite of where it was then tighten it up real good again and leave it there.
it kinda crimps the exhaust pipe the whole way around if you get my meaning