* The sticker on the driver's side doorjamb: This sticker lists the recommended tire size and inflation pressure. This is the most reliable source.
* The owner's manual: If you have the original owner's manual, it will specify the tire sizes for your specific truck.
* The tire sidewall: The size is printed on the sidewall of your existing tires (if they are original or correctly sized replacements). This is only useful if the tires are already correctly sized.
Without checking one of these sources, it's impossible to give you a definitive answer.