Stock rim size depends on the package you got... if you know the chassis code: V44WG (or 24 if you're super cool and going with a shorty <img src="/forums/images/graemlins/smile.gif" alt="" />) means it should have flares, and comes stock with 31x1050s on 15x7 rims. Otherwise it may have p series on 15x6. Unless it has aftermarket wheels...

Anyways, with the flares, the wheels have a slightly bigger offset, and I put 33x1050s on my 15x7 rims with no modifications at all. If you're getting a stick, you could end up with 529 diff gears. Mine shipped with 488s, but even with 33s on there, I'm at 3500rpm at 120kmh. Needless to say I try to keep it below that. 33s made my truck much more driveable on the highway.

Any pics of your rig?