What Class...?? Or did I read somewhere this was just for banging around...??
Some classes have restrictions on sizes and you would need to go by the rules....If the gear ratio is "fixed" then get the "tallest" tire (brand/circumference) you can find in the regulated tire size...
If you gear ratio is open, a shorter tire will make the mowchine more stable (lower to ground) and be able to "spin up" faster (better leverage)...