Bit on the small size for a woofer I think he means.
I am running 16cm Kefs and they fit a treat though they are not in the car at the mo - developments... Try to get a driver with a decent motor assembly (i.e. big basket/magnet) - cast baskets best way to go for decent bottom end with no sub. You'll also want to make sure the speaker is well-fixed to the trim panel but preferably also to the door frame, and the door 'enclosure' is as airtight as you can get it for best results, regardless of whether you are running the speakers straight from the head or via an amp.
Whatever speakers you fit, as long as you are not making big door mods the finished project will not have a negative effect on value, as most cars have been through a few sets of speakers since they were new, so don't get too worried if you have to trim a few bits back. Use new grilles too - tidy grilles look good and are usually matched to the speakers from a fitting point of view.