speaker recommendations

Many choices here. www.parts-express.com
I put dual four inchers in my Barracuda and Valiant. I even found room to mount small tweeters in the Valiant with the crossovers sitting on a bracket on the radio. The Barracuda has coaxial two-way Boston Acoustics. Both cars sound good and have surprising stereo separation bouncing off the windshield. Not great, but better than you would expect from speakers so close to each other. As BrianT mentioned, if you do it you can prevent over driving the small speakers by mounting a filter capacitor. It keeps the small speakers from trying to create too deep of frequency and buzzing or popping due to lack of control...maybe even burning up the driver coils.
496 MFD blocks below 80Hz @ 4 Ohms
398 MFD blocks below 100 Hz @ 4 ohms
330 MFD blocks below 120 Hz@ 4 ohms
This is an approximation so capacitor values can be slightly different.
You can hide some 6" or lager subs somewhere else in the car if you really need the bass of a larger driver.