Speakers that fits would be 15 cm.. or 6".
A very good alternativ is the Alpine lineup. Right now Alpine has a 2 componet speaker set on sale(in Denmark). It fits perfectly in the rear.
Use it without the high frq. unit, as a small subwoofer(use a simple crossover, a spool to filter out the mid- and high frq. haven't got the size in mH, for the spol, of the top of my head) combined with a quality 2-way unit in the front. Again Alpine has the stuff 10cm or 4".
This makes very good sound in the front of the car, but obviosly not that good in the rear..
Naturaly you have to have a desent headunit or better an amplifier..