you cand do 27's with no interference at all, and 28's with possibly a little rubbing on big bumps. The 500's really dont need a lift with any tire, i know of people running 29.5's just by trimming the fenders and adjusting the fender rods.
I just put 27's on my 06' and they won't rub unless there is a lot of weight on the back. I am probably going to trim the finders about a 1/4 just to make sure or I may try adjusting the finder bracket like someone else said.I am looking at putting a Big Foot Kit on my 2006 Foreman 500. I am very interested in the Mudlites on Type 5's. What is the biggest I can go without having to put a lift kit on the bike?
Try using a heat gun to bend the black plastic out a bit, that may be all the clearance you need.I am probably going to trim the finders about a 1/4 just to make sure