The stock size is 4x6, but if you look on crutchfield, they suggest a 4" or if you buy a bigger bracket and don't mind trimming a little bit, you can stuff a 5.25" in there. Nothing that small is going to give you good bass imo.
When I replaced my speakers, I put in new 4x6s, but if I did it again I would go the 5.25 route.
Car Speakers FAQ
It should cover most of your questions on what is a good speaker, or what to look for.
I have something like these in my jeep (or this price range at least, I don't remember what exactly is in there): Alpine SPS-406 Type-S 4"x6" 2-way car speakers at Crutchfield.com
The mounting bracket for the stock speakers is HUGE, so I had to cut the old molded speakers out and use self tapping screws to hold the new speakers in.