camoford, how wide of a rim are you planning on running? This will drive a lot of what the "max" will be. I am running 37x12.5's with only a 4" suspension lift and you could probably get away with only a 3" suspension lift. But, to get this setup, there is a lot of very specific stuff you need to do. In short, you want the maximum tire, 8" rim is the only way to go. Going wide and you are going to have rubbing issues. TRUST ME!!!! Go narrower and you are going to be limited on how wide of a tire you can run, which is then going ot limit the size tire you can run. Also, 4.5" of backspacing is VITAL!!!!! When you are done, you are going to have less than 1/4" from tons of different things on your truck. If you want a little bit of breathing room, then 35x12.5's on an 8" rim will be your best choice.
If you are looking at more than 35" tires, be ready for some other mods. I managed to squeeze the 37's on to my truck with very minimal spent in fixing the other issues to make the tires work. But, you have to also ask yourself if you trust yourself enough to take a 3 pound hammer to the body of the truck? That is what it is going to take to go that big. Then you also have to trim some of the metal off of the bumper to make room there.
How big do you want to go and what are you willing to spend? You can run up to 35's on the stock 3.73 gears with no major issues (just have to limit how much you put in the back of the truck and can't be pulling any trailers over 5,000 pounds. If you have 3.55 gears, then I would say 34's are as big as you want to go. Now, this all goes away if you regear. But, that is roughly $1000 per axle.
If you need to know anything, let me know. I have been down this road before and I can tell you what will fit and what won't.