15" has a much wider selection of tires, that are less expensive than the other rim sizes.
7" wide rims will take 10.5 tires easily...most people like an 8" rim to go 12.5 wide. Backspacing is critical on those, tho.
31's will fit on a street duty stock YJ if you trim your flares a bit. They rub slightly when turning very sharply, but you can reset your steering stops to avoid that.
Off road, you can run 31s if you go to TJ flares/1" body lift & trim a bit of metal exposed by the new flares....
probably out breaking stuff.