When I purchased my first set of staggered GTR's I ran 265/50/20's front and 295/45/20' rears it worked very well (but only AFTER I had the rear wells rolled). Then I bought another set of wheels and ran 10.5's on all four corners with 295/40/20's. The 40's are really nice as they are lower sidewall's (but not too low), they still have enough rubber for a decent ride, and the rears cambered in nicely as the FX dropped. IF you run lower sidewalls like the 40's you are going to want to get coil overs or you will have a fairly large well gap.
I am really happy with the look. It took awhile to dial in but it was worth it.