It varies from room to room, and is no different than m80's or other ported speakers. You will have to experiment, but I would say you should allow at least 6-8" behind the speaker. Also, I would consider that you may upgrade some day, and you don't want to restrict yourself if you get M80's or other speakers.
Keep in mind the BIGGEST screen is not always best, it depends on how far your viewers will sit back from the screen. My screen is a 2.35 aspect ratio and 120" wide, people in the front row, 13ft back, often say that fast action movies are hard to follow as your eyes dart back and forth. Make sure you think about everything...
Maybe it would be better to not use a transparent screen, is there a reason you want to hide the speakers. It may be just me, but I don't want to hide something that you pay that much for and looks that nice. Have you considered the M60 cabinet speakers?
