I wouldn't go bigger than 26s w/o serious mods. As far as width, you can run Vampire ASXs (26 x 13.5) in the rear as long as you have the 12 wheels for them. As for the fronts, stick w/ 8-9 inch wide, since its full time 4wd.
Did the 12's rub at all? Thinking about 25x12.50x11's on the rear of mine.i just put a new set of 27x12x12 mudbugs on my 01 rancher and it has no mods really. no lift or anything. turns them not bad but should have some mods to turn the tires in thick mud,