I have a 1996 F-150 Regular Cab. My door speakers are 6.5". The rear speakers are supposedly 6" X 8". I have not seen the rear speakers, but I have fooled with the door speakers. drteater is right. Check Crutchfield. It shows 6.5" for 1992 Regular Cab, Super Cab and Super Crew. Maybe cgorris is thinking about 1997 and newer. It shows 6" X 8" for those.
If you are thinking about changing speakers, you may want to buy a pair of Metra speaker wire adapters. I bought a pair at Auto Zone for $6. They keep you from having to fool with cutting the original wire and crimping on a new terminal, etc. It makes it a whole lot easier. It is the 72-5512 model. It may show a little different part number at Auto Zone, but the final number will be 5512.
Hope this helps,