I have a couple things that might help you make a better decision having owned two Grizzly's.

1. Buy wheel spacers or buy some nice rims with a greater offset. Most aftermarket rims have a bigger offset. This will give your quad a wider stance and virtually eliminate the tippy feeling most people complain about on the Grizzly.
2. My own preference is to stick with the stock size tires. Taller and wider tires will put more stress on the drive shafts and it will not perform as well. The Grizzly will definately handle any size tire you put on it without too much problem, but they will make the machine handle differently. I like to ride fast. My buddy's Grizzly with 26" Blackwaters runs about 3 miles an hour slower and is slower off the line than mine with 25" Mudzilla's.

I wouldn't recommend the Mudzilla's though. They ride a little rough at slow speeds and do not climb rocky terrain as well. Any of the tires listed here would be a better tire.