Depends on the condidtion of your springs, which wheels you have, and what you plan to use the Jeep for.
30" and even 31's will fit stock 15x7 wheels but will rub while offroad, as well as rub the leaf springs when the wheels are fully turned.
With stock steel 15x6 wheels you'd be best served staying with a 235x75x15 or smaller.
Personally, I wouldn't go bigger that 235's without some lift no matter what the wheel size. I'd prefer the room for wheel travel over 1/2 - 1" of extra ground clearance the larger tire size would give me.