the port size will depend on your tuning... bigger ports mean higher tuning.. low tuning usually means small ports. so that is working in your favor. If you plan on building in the back seat then i would build one of two ways, subs up ports forward or subs forward ports up, or the third way... get the darkest tint you can and build a wall in the back seat! that would be hella bad ***...

 

I had to seperate boxes, 2 subs in each. They were about 3.5 cubic feet with two 3" ports at 12 inches long in each box.
I was running 2 optimal batteries. But if your going to run more then one battery I'd suggest upgrading your alternator, I was replacing mine quite often.

 

You can get 5 cubes in the trunk, from the inside of the car, anything more you will have trouble. Youll have to build the box in the trunk.

 

4cu.ft. net box will go in the trunk easily. If you check my SD page there's a couple setups from my old Sunfire. One was a single 15" in a 3.5net and there was easily another 1.0 net available for a slide in box.
Another shot is a pair of ported 12's.
I would recommend you either do a single 15" or a pair of 12's. Ported of course with ample power. Pick the right gear and enclosure and have an electrical that can support it and it will be all you need.

 

The Sunfire was stolen and set on fire Sunday night. Looks light I might be getting my hatchback afterall. They took my Pioneer 7" flip and my Atomic APX 12 (in a black carpet slot port). There is a code lock on the deck anyways so they can't even use it. They also hit a few cars before setting it on fire, and the idiots threw my paperwork under one of the cars, and now it's being dusted for fingerprints, what a bunch of morons!

 

That sucks!
Just an FYI, in case you buy another J-body. You can cut out the angled top corners in the rear seat opening and slide a rectangular box into the trunk (max of ~7cuft NET can be had). Dozens of people have done this with no reports of structural issues
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Single 15s are as easy as cake to install in a Sunfire. Dual-15s (or 3+ 12s) are more difficult, as the trunk opening isn't wide enough to have adequate port area on the same plane as the subs (unless firing everything upward).
Believe it or not, I've wedged an 18" CV Stroker in the back of my 2001 Z24, rear-firing no less. Had to angle the baffle considerably, but it was (barely) doable. Biggest problem I had was that the angled baffle used up much needed airspace for the Stroker. In the end, my single DD 9515 setup destroyed it.

There is a fair amount of info on the j-body.org - the jbody organization website. Not that it really matters at this point with your car getting torched, but the info is there if anyone needs it.

Good luck with insurance! I've never had a good experience dealing with insurance companies