super swamper tsl work awesome for me (22" on a highly modified warrior) good in the mud ,decent trail tire as for the ride. also look into a outerwear for your filter they help stop water from getting into your motor. also the only thing is the swampers tend to swell a bit at around 80- 90 mph on the road (very shakey ride)

 

A bigger rear tire will change your gear ratio. It will gear it up, which is good in mud. But i would'nt go too much bigger.