largest tires for 2008 H2

[ken.j]

I have just got my 08 H2 and would like to modify, starting with wheels/tires.
It has stock 20" with 305's and would appreciate knowing how wide and tall I could go on these stock rims without rubbing issues or sacraficing ride quality or as I do a lot of highway driving. Also if I ditch the fatory rims and tires again what is the widest and tallest configuration available. I want the truck to maintain its agressive look so don't want the wagon wheel no rubber rapper look - prefer to stick with 18-20" rims. Would appreciate any and all suggestions including rim/tire brands, site links or pics of other forum members rigs.

Thanks

Ken.J