Mikey7700

i have an 08 FX4, with the 20" factory wheels. (7.5" wide i think). im also told it has a leveling kit on it(not officially confirmed yet) i was wondering if i could step down to an 18" or 17" wheel and maybe go up to a 35 for tire size. Either a mickey thompson baja mtz or a cooper discoverer stt. is this possible? if so what back spacing/offset should i get on the wheel? or do i have to stick with a 33" tire? any help would be appreciated.