A member in Woodland Park Colorado! WOOT! You're just a stones throw from me. What do you have on there now? 265/70/15? The aforementioned size is what mine had when I got it, and is what the factory states is the size the vehicle needs IIRC. It works out to be about 29 inches in diameter by 10.5 inches wide.
A hair bigger should be ok, as kefferjoe posted, a 265/75/15, but I don't recall that size being very common around here, you might not have a lot of choices of brand/tread pattern.
Perhaps a 30/1050/15? *edit Nope that size is even less common, but it was one of the search options so I know they exist.
I put 31/1050/15's on but I got new rims with the needed backspacing. I'm still waiting to gather the last pieces for my lift, but it works still.