according with the fact that you can mount your screen directly on the wall, andyou can mount the pj wherever you want on the ceiling (== it is totally free for this purpose), I don't think that you will have any problem obtainign whatever screen size you want, within the limits of the 3CRT technology (some 9-10 feet screen base)... with all the projectors on the market, I think.
You might want to do a search for past threads on this--there's LOTS of info on this, and a good bit of disagreement.
To recap, screen size for CRT FPTV-based theaters (i.e., ones that use a front projection CRT display) is a factor of the light output of the projector, the ability to control light in the room and the desired "brightness" of the picture, as measured in foot lamberts (ftL). The same variables also hold true for non-CRT displays, but remember that CRTs have--on average--lower light output than their digital bretheren (e.g., LCD, DLP, DILA).
In a 100% light controlled room, you want to shoot for 12-14 ftL, which is about the same as you would get in a good quality moview theater...the 12-14 guideline is hotly debated in the digital projector forum, but I think most everyone would agree that it is a good place to start. With a CRT FPTV, it's really hard to get there without considering a smaller screen. Others will argue that they get great results using a 7" CRT on a 100" wide screen.
The formula for deriving what ftL you'll see with a given setup is:
(Projector ANSI Lumens) / (Screen Area in Feet) * (Screen Gain)
Say you have an 8" CRT FPTV that puts out 240 ANSI lumens and a 16:9 84" wide 1.3 gain screen--that would give you about 11.3 ftL...and likely a great picture. Moving to a 96" wide screen would drop you to 8.7 ftL...where some would argue that you're dangerously close to the lower limit of "acceptable" and others would say you're still fine. Your mileage may vary!
As for the relation of screen size to room size, this has also been discussed ad nauseum as well. The first key variable is how far to sit back from the screen (the "optimal viewing distance")...it is usually source dependent (you can sit closer with HDTV signals than with NTSC signals, due to resolution, artifacts, etc.), but unless you plan on installing your seats on a sliding track, you're going to have to select a happy medium. Then you have to factor in audio--room dimensions impact peaks and nulls.
I have attached a little Excel spreadsheet that I find somewhat helpful in trying to figure out everything, including "optimal" room length for audio if you sit at the 3/5 point or the 4/7 point, etc. Just remember that optimal is only a starting point or a simple guideline--this is in no way a replacement for sound HT design, it is only for rough guestimates. Trust me, my theater design doesn't jibe 100% with this, but this spreadsheet did help me figure out what was and wasn't possible with my room.
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