Some came with 6x9 and some came with round speakers - gotta look and see. My 88 vert has the round speakers. Didn't take the quarter panel off yet to measure them. Used an inspection mirror and flash light with the window down to take a look-see.
Pre-90 vert's are round 5.25"'s and are at the bottom of the quarter panel. I squeezed in 6.5's with minor modification.
90+ cars have the speakers in the middle of the quarter panel, allowing much larger speakers. Stock may have been 5x7, but if you watched the mounting depth im sure you could fit in 6x9's. Remember that when you buy speakers, mounting depth can get you.
Also when I was looking around someone showed me some pics he did of a custom vert speaker installation. Basically, he made a bracket to hold the large speakers where the armrest is on the quarter panel. I didnt want my speakers out in the open, but rather enclosed like stock, nor did I want even LESS room in the back seat area!
Not to mention if your vert. is regularly driven you wouldnt want someone ripping off your speakers...
I put 6x9's in the back of my 88 GT vert and it was A PITA! Lots of massaging and dinkin around to get the panels to fit flush. I'm gonna actually swap them down to something smaller when I deal with my rear window motor issues because I dont want to spend 2 hours trying to get the panels semi-flush again.
FWIW, if it's an early aero vert, when I drilled the PITA rivets out (holding the speakers), I had to pull the carpet up (on the plastic panel). Have some 3M adhesive on hand to meld the carpet back onto the panel.