With a 15 x 7, 255/70/15 is about as far as you can really go without getting into tire support and handling problems. That tire as about 10.4" wide, should fit fine with the stock width axle and proper backspace wheels. I would not go any smaller than a 225/75/15 on a 7' rim, any narrower could get into a "ballooning" issue with the tires, again causing a tire suppoer and handling issue. There's also an issue with lack of load capacity with undersize tires. While you do mention that you don't plan of hauling anything, you do need to bear in mind the empty vehicle weight plus passenger/cargo weight when deciding on tires. I strongly advise staying away form any low profile tires, which have a greatly reduced load capacity.
