Look inside the gas door or on the drivers side door jam it will tell you there. If it doesnt take your vin number to the dealer and they will tell you. That is the ONLY way to know. Ford puts so many different sizes of tires on trucks that its a crap shoot to guess what was on there from the factory.
I work in a tire shop and when someone is looking for a Ford truck size I just go and look at the vehicle because the computer isnt going to tell you.