2000 LS. Door speakers are 6x8 I believe. Will any other sizes work without modification? Some speakers i've seen advertise as 5x7/6x8. I don't have many options around my area so i'd like to go 5x7 if possible. or maybe 6 1/2's.

Yes some 5x7s will work. I have some 5x7s in my car and they fit perfectly. You can go 6.5 also, some ppl believe the round speakers sound better, but for those you will need an adapting piece for them to fit.

You can fit up to a 7" woofer in the door without any major work. I installed Aluminum Dayton RS drivers in the doors. They sound awsome. I just had to make a thin wood mounting plate. (You can trace the mounting hole locations from your stock speakers onto some plywood or MDF. Then just cut the appropriate size hole for the speaker you want to install and wala. Perfect fit)
Directed Audio does make an excellent 5x7 Neo component set as well that would drop right in. Ive seen them cheap at mainstreet-audio.com

That's a great idea. I just put Directed Audio 5x7's in the front. A big difference compared to stock. Those subs would sound pretty good in the back i imagine. Thanks!

What speaker size is the back??

edit: slow computer. double post

In mine? Still stock. I'm waiting for something i really like. Also, still deciding whether or not to put a 12 in the trunk. I have a Pioneer 12 (entry level) in a bandpass enclosure from a previous car that i was gonna try to hook up to see how it sounded; but since i don't have the Alpine stereo, I don't really know what's involved to wire it.