The base sound systems had 4x6s in the front, the uplevel ones 5.25s. The easiest way to tell is to look at the door panels, if there is one grille its a 4x6, if there are two grilles (one large one for the speaker and one small one for the tweeter) its a 5.25.
GM has some of the worst-designed interiors I have ever had the pleasure of working on. To get at the speakers you will have to take the whole door panel off. There are a bunch of torx screws (another stupid GM thing) around the perimeter of the panel. You will also need to pop the lock thingy off and the couple little plastic panels behind it, there'* a screw behind them. Next, take off the plastic piece that the power mirror switch is mounted on, there'* a bolt behind that. And finally, pop off the rectangular light bar that are on the lower right off the doors, there are a couple bolts behind those too.
If that wasn't enough, there are plastic plugs all around the perimeter of the door panel, these can be popped out, but be careful that you don't break any. Go slow.
One more thing, I don't think 5.25s will fit in a 4x6 space. I had 4x6s, and from what I saw, they wouldn't fit without a special mounting bracket (which I doubt anybody makes).
1992 Pontiac Bonneville SE, 163k miles - RIP (blew the transaxle)
1995 Honda Civic EX, 5psd