Typically, around a 245 for 8.5"; 275 for 9.5", but there is usually ~ +/- 0.75" tolerance. (my 275s are on 10") Sites like The Tire Rack have complete specs, including recommended rim widths)
Well, my 275s have never rubbed at the front (occassional light scuffs on big bumps with full loads at the rear) with about a 1" drop, so I think 245s would be OK even with a 2" + drop, but for sure 235s work; several members are using them. (a 245/40 x 17" is nearly dead-on the stock diameter)