Grab an offset axle, you can run it up to 8" or 9", you can go too big and run into problems, I personally like 8's. If you want to stick with the stock axle, you have to stay with the stock wheel size which is 7 something.
up to 10's if you want rails and custom offset. my buddies run 8's. I run 9's and will never run anything else. the reason i do big wheels is cause i can back my sled up in 3feet of fresh and not instantly trench down. helps out in alot of situations