* The sticker on the driver's side doorjamb: This sticker lists the tire size and sometimes the wheel size.
* Your owner's manual: This will specify the recommended tire and wheel sizes.
* The wheels themselves: The size is stamped on the wheel (usually on the inside or back). It will be a number like "16x7" which means 16-inch diameter and 7-inch width.
Without knowing your specific 2002 Ford Explorer's trim level and options, I can't give you a definitive answer.