6.5" is what I used. I just did this in my 91 last weekend and I'm almost positive the speaker size stayed the same for the next gen body. Unless they added additional speakers with the body change, the door speakers are all you have if you have a regular cab.
The only issue I had was that the mounting holes in the speakers didn't all line up with the existing holes in the door (2 would, 2 wouldn't no matter how I clocked it), but a couple seconds with a drill took care of that. I drilled the door, not the speaker, btw.
To make life REALLY easy on yourself, pick up a set of these:
They click into the stock speaker harness and keep you from having to clip off the stock harnesses (you know, in case you ever want to install those LOVELY OEM speakers back in). The REAL advantage is that it quickly gives you a couple more inches of speaker wire to mess with. You'll find that Ford was none too generous with extra slack in the speaker wire, so these help a LOT. Not necessary, but I really like them. If you get lucky, the terminals at the other end will slide onto the connections on your new speakers. I didn't have this luck with the Pioneers I installed, but all I had to do was cut off those terminals and crimp on the wires that came with the speakers. Easy as pie.
It should be noted that these are what I used on my 91. I would assume, based on Metra's description, that the 92 would use the same style connector, but if not, I'm sure they make one for your application.
Would the speaker size be the same for a 1992 f150 reg cab long box 2 wheel drive? Looking to do the speakers in my truck as well.
Yes door speakers stayed 6.5, and check to see if you have rear cab speakers, in the single cabs I've heard of some people having rear speakers and some not in the single cab, my 93 and my 94 both have rear speakers and there both single cabs.