NOT SURE WHAT YOUR TIRE SIZE IS? THERE ARE TWO GREAT SPOTS TO LOOK:
Firstly, there will be a set of 3 numbers printed right on the outside of the tire in a similar format to P215/65R15.
Secondly, there should be a sticker on the edge of your driver's side door when you open it. Within everything on that sticker there should be a 'Tire Size' column or row with a similar 3 number code as above. That is your original/stock tire size for your vehicle. Some may show numbers for the front, back, and spare tires like in this example: