How big of a speaker that you can fit under the seats depends on how much you are willing to raise those seats. A friend of mine used 3" spacers I believe and longer bolts to fit two Memphis twelves with over 1.25 cu ft of air space per speaker. Lifting the seats that much means that when I sit back there my head touches the headliner (I'm 6'1"), but it does hit low notes really well and he doesn't have to ride back there either.
Another thing to consider besides air space is the mounting depth for your desired speaker. I am about to build an enclosure for my truck and do not want to lift my seats much, if at all. Speakers like Image Dynamics IDQs, Apline Type Rs and JL Audio W3s all work well in a ~.7-1 cu ft box and have adequate mounting depths for such an enclosure.