Dark-Rider--thanks!
SupraMK3--
Actually, the rear speakers are 4.5". I added some more pics. Go back and take a look. I used a dremel on the rear speaker boxes to fit 5 1/4" speakers in them (Infinity Kappa component, haven't decided where the tweeters will go). I don't see how you could get anything larger than 5 1/4" in the boxes. One thing though, watch your speaker depth. The boxes will only allow a depth of 2" - 2 1/4". I listened to some RF speakers, but wasn't very impressed. Don't know anything about the Eclipse's, but have heard they're nice. If money were no object, I'd go with Focals.
The rear seat speakers measure 3 3/8" (see the new pics). I'm going to rig a custom mounting bracket and put 4" components in their place. Yeah, it's probably overkill, but I have the components for free, so I'm going to try it and see how it sounds. The other option is to just disconnect them.
For now, I just replaced the front speakers with 4" components. The mids fit perfectly in the speaker box. I still haven't mounted the tweeters, but I'm leaning toward mounting them on the little black plastic piece at the front of the window (by the mirror).
You can fit 6.5" in the door, but you'll have to toss the speaker box and make a custom bracket to mount them. If Needsboost would ever get us those damn pics

, we could see how his are mounted!! He only has the 6.5" in the front, nothing else.
The main thing about your stereo set up is that you're happy with it. There will always be someone who thinks your set up sucks or could be better ( and maybe it could if money were no object).
My stereo installation is very much a work in progress. Eventually, I'll get all of this hooked up to my RF Power 800.4 amp and add my 8" powered sub. I got sidetracked by the water leak that I discovered when I pulled all of the plastic out of the hatch area. (Tail lights were leaking). I've had some serious rusting in the spare tire well that I had to treat before I could go any further. I'll put the final coat of paint on it tomorrow.
Next step will be to line the entire hatch and back seat area with sound deadener before I put the plastic parts back in.
Ditto for the door panels. Hope I answered your questions.