Depending on how you have your Bronco set up, a set of 32 x 11.50's, or maybe even 33 x 11.50's would work perfectly, look good, and fit without lift on a cut Bronco. Pretty tight on the rear of an uncut, but if the fenders are cut they're a perfect size.
I've always used the same rule-of-thumb that casadejohnson spoke of. Tires should be at least 2, but usually not more than 4 inches wider than the wheel. Plenty of hard-core users and, evidently, members here, using 12.50's on 7's though. And very common practice in the old desert racing days, so you at least know that it works.
As always, you adjust your air pressure to match your vehicle/usage/wheel/tire combination.
Paul