you dont need to go full on 12" to get a decent bass sound out of any vehicle. 2 8s would fill in nicely, and fit in a lot of spaces. that space you have there is perfect for it. if you wanted to go for more, toss a 10" in there and use the whole area as a chamber for it. dynomat that sucker also.
the stock stereos that say 50X4 are usually about 23W rms, and will do fine with smaller speakers. if you get a decent set of speakers, you should get a 4 channel amp with at least 50W rms to drive them, the sound is so much better. also, you can damage good speakers with no amp either, usually by turning them up higher. ive ruined a few sets of memphis components now, ive been too lazy to install my amp in my truck.
also, that custom speaker set up in that pic is nice, any reputable sound shop can make that to spec for ya, whether its fiberglass or mdf. my friends custom shop does some pretty neat work for enclosures, the latest was a coffin-shaped box out of oak panel. its in the back of a pt cruiser that is decorated in skulls.