* Check the sticker on your driver's side doorjamb: This sticker lists the recommended tire size and inflation pressure for your specific vehicle. This is the most reliable source.
* Check your owner's manual: The owner's manual will also specify the recommended tire sizes.
* Look at the sidewall of your existing tires: The size is printed on the sidewall of your current tires. This will tell you what size is currently on your vehicle, but it might not be the optimal or recommended size.
Without knowing your specific Explorer's trim level (Sport Trac, Limited, etc.) I cannot provide a definitive answer.