What size subwoofer to get?

In most cases I recommend getting 2 lesser subwoofers over 1 larger one. It is much easier to place and avoid any major peaks and nodes in the room. These peaks are generally what is perceived as boomy and ill defined bass. If you use 2 you can smooth out the subwoofer to room response. The size of the subwoofer usually determines how low the unit will go. If you are looking for those last octaves, you will likely choose one of the larger ones. The other word of caution, particularly in 2 channel, is integration. Some subs sound great with speaker A but very poor with speaker B.

Gunbei: It's a good question, but fortunately you almost never need to compromise. The flatest response is with subs placed on opposite walls at the midpoint. Now if it were for music and it was a high crossover point (say 80Hz or higher) I would move them back closer to the main speakers but symetrical, and relatively close to the wall (within 1/8 of a wavelength of the crossover point). In a non-symetrical room, this may not be the case. Here's an even more interesting phenomenon (purely hypothetical--because I don't know anyone that's actually done a room like this). If you use 4 subwoofers and place them 1/4 of the crossover wavelength from the corners of the room you get a nearly flat response throughout the room--no peaks or nulls (obviously there would be minor ripples due to furniture and other items in the room in practice, but in theory it's a flat response). In this last example it does assume a mono signal. In your case, you would not need a mono signal because most of the lower bass notes are mono. If the crossover point is high--then that might not be true and you could run into some difficulties, but no more than having 2 main channel speakers that carry the bass information independently. So, I guess I don't really see any significant downside to stereo subs--I hope I've explained that adequately.

Gunbei: I think you are headed in the right direction. Subs in the corner behind the speakers can and often do work quite well. It's not as flat a response, but if you keep them within 1/8 of a wavelength of the corner it minimizes the peak/node effect to some degree (eliminates the re-enforcement of back waves coupling or decoupling to the source wave. As to downfiring or forward firing--it makes no difference at crossover points below 65 Hz or so. Above that you might get a very slight difference. You may want to go to our website www.rivesaudio.com and go to the listening room (under acoustic issues). If you click on speakers there, there is a link that you can download a white paper on speaker placement. It goes through all channels and subwoofer placement. Hopefully, it's helpful.