If you could afford a denon 5805 in the first place, you should be asking, 1 extremely large sub, or 2 just large subs.
But to answer your question, I would get 1 medium sub for now and if your taste for bass increases then you can just simpley add another medium or even a large sub later on.
If you want a small sub because of placement and wife issues, I would recommend getting a velodyne splII, or a paradigm siesmic 12. Small box with big sound. Both of these subs are costly though.