* The sticker on the driver's side doorjamb: This sticker usually lists the tire size, which will indirectly tell you the wheel size. The tire size is written as something like P195/75R15 or similar. The last number (15 in this example) is the wheel diameter in inches.
* The wheels themselves: Look at the side of the wheel; the size is usually stamped there. It will be a number like 14x6 or 15x7.
* Your owner's manual: This should specify the factory recommended tire and wheel sizes.
Without knowing your specific S-10's configuration, it's impossible to give you a definitive answer. Common sizes were 14" and 15", but other sizes might have been options depending on the trim level.
