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:

Metra Part # 72-5512 | Ford wire harness | Metra Online

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.

Mike