This is just pulling everything from memory. Factory: front 5.25, rear 4.
Depending how much you modify with/without spacers and/or cutting, you can fit 6.5" in the front AND rear. I know this, because I did it. The only reason I put 6.5" in the rear was just to say I did since it was my part time job. It's not really necessary to have rear speakers that size.
Fitting 6.5" in the front wasn't too difficult. Getting the crossovers and new tweeters mounted was a little bit of work. Cutting all that metal for the rear door with an airsaw for such big speakers was a PITA. Wish I had ear plugs at the time.
"We live for a good time, not a long time."
RIP Colin McRae