Lots of stupid in this thread
Stock speakers in a CE:
Fronts : 5.25 inch
Rears: 6.75 inch
Yes I am serious
I have fit 6.5" speakers in both spots.
If you get them seated solid, they will make good bass, provided they are decent speakers.
Look at the frequency range they are quoted to output on the spec sheet. anything that goes lower than 40Hz will have pretty sweet bass. Although nothing like a sub.
If you don't want a loud system, you don't need to amp the door speakers. They will sound fine running off the headunit. But trust me when I say you will want to turn your speakers up more when they sound good.
Turning up speakers that are running off the headunit too much can start to damage them. You get close to the limit of the output of the headunit, and this is where you start to damage speakers - not enough power. Adding an amp fixes this. Sort of counterintuitive, but it makes sense once you get it.
Btw, my system was extremely cheap and I am really happy with the sound of it. I bought a refurbished Pioneer headunit off ebay, Polk Audio db651's for the front, some type of pioneer 6.5"s for the back, and an alpine type s sub with an alpine amp that is something like 300w. If you don't want loud, it's awesome.