Towing capacity will depend on what trans you have, what rear end ratio you have, and tire size.

However, the real limiting factor is going to be STOPPING the trailer, not pulling it. The F150 does not have monster brakes, so even if you had a 460 with 4:88 gears, you would not want to pull a really heavy trailer. Personally, I would not want to pull anything over about 4000 lbs with F150 brakes.