Mine's a '92 and they're 5 1/4

Pretty sure it's the same on all body styles.

You can easily fit 6 1/2 speakers in the back if you want, but you'll most likely have to be content with having only 1 (or 2 max) bolt holding them in place, or drill some new holes - being careful not to come down on any wires.

It would help if you got speakers with multiple mounting holes, instead of just three or four.

Even then you might want to get some padding to put between the speaker and the metal it's mounted to so you don't have issues with vibration.

edit: h: Or, apparently, you can just order them with the proper mounting brackets that will make almost anything fit. On the site that Chefboiali provided h: