* Check your owner's manual: This is the most reliable source of information.
* Look at the fuel door: Sometimes the tank size is printed on a sticker there.
* Check the sticker on the driver's side doorjamb: This sticker often lists various vehicle specifications, which might include the fuel tank capacity.
Without knowing the specific trim level and options of your 2001 F-150, it's impossible to say definitively which size tank you have.
