Perhaps someone can confirm this:
I removed the grill and I saw what looks to me as a 5.25" speaker. A friend of mine worked at Circuit City said that the 323 has 6.5" speakers all around (Front doors and rear panels) and that's great if it is, but looking at the speakers its looks rather small to be 6.5" speakers. Also, has anyone put polyfill in their doors or should I just get a roll of sound deadening from McMaster? I have also heard that if the speaker is too large, the window won't crank down all the way. Any help/advice would be cool.

 

5.25" fits best (though not perfectly)
If the speakers are a bit deep, take a bit of 3/8" plywood and make baffles, that way you can get up to about 2.5" of clearance.

The rear speakers that fit best are 6.5" but the screw holes are weird so you will have to make baffles anyways.