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I think you'll be limited by how wide a tire will fit on the factory 20's. Rims with a lower offset or wider rims will make the biggest difference.
Your question of how wide you can go I don't know :P
The rims are 8" wide from the factory, reasonably any tire 11.5" and narrower will work.
You will have to look and see whats out there from this point.
STEVE