Well, wheel size is kind of a personal thing. If you regularly spell "wheels" or "rims" with a "Z", I'd recommend a 26" or a 28", spinners if you can find them.
Otherwise, my personal opinion is, for an off-road rig, the rim should be a litle less then half the tire dia. Much taller sidewall and you start running into tire carcass stability issues. Take the the tire dai, subtract 1 inch, then go with the nearest whole size. With this formula, I'd stick with a 15" for anything up to a 32" tire.
As for width, the rim should be slightly narrower then the tread. Not the tire body, the tread. For example: A BFG 30x9.50-15 has an actual tread width of 7.5 inches. A 7 inch rim is fine for this tire. An 8 inch rim would be a little too wide.