If your wanting to get bigger, I just installed 27x11x12 itp black waters all the way around on stock wheels. Looks great, not loud and around $146 each delivered. They also have a 1k lb weight rating.
As Brett mentioned, pay attention to weight capacity rating. I've not verified this but would expect the rears to be carrying 600lbs each when the RTV is empty and driver-less.
I keep it in the basement garage in winter, its heated, I have about 2" of clearance with new OEM's, so I can't use a taller tire.
I never thought about the weight rating. I'll weight a little over 2000 lbs w/the Kubota RTV cab add-on.
I just found a weight rating for Spartacus ATV's, 595 lbs, guess I'll keep looking.
Thanks