* The sticker on the driver's side doorjamb: This sticker will list the recommended tire size and inflation pressure. This is the most reliable source.
* Your owner's manual: The owner's manual will also specify the recommended tire size(s).
* The sidewall of your existing tires: The size is printed on the sidewall of each tire. This is only helpful if the current tires are the correct size.
Without this information, it's impossible to give you the correct tire size.