Having worked at a shop, there were actually 3 sizes of speakers used.

base RS cars had 5.25 all the way around, this was if your car originally came with a radio, maybe a cassette player.

RS with some options (power doors) may have had 6.5 in the front, still 5.25 rear

GS usually had 6.5 front (I have seen 5.25) and an oversized 6.5 (more like a 6.75, but noone makes a 6.75)

turbo cars usually had the same as GS, but occasionally the Infinity package sound system had 6x9 rears. infinity also used the speakers in the dash (there in every trim level but not hooked up for some stupid reason)

a Dremel will help if you want 6x9 in the rears, but don't go too big on magnet size unless you want to pound the inner fender to fit with a hammer

Erik P.
Official 2GNT thread Hijacker
Some say that he's driven over more Covenant than he's shot, and that his grenades aren't where you'd expect them to be...
All we know is, He's not The Stig... But he is The Stig's Spartan cousin!