Nick, something you might want to try if you haven’t already is to get a couple different colours of painters tape and using the calculators Murph linked to tape off the screen sizes for the aspect ratios on the wall to get a better idea of what the visual impact of them will be for various seating positions. When I did this it made the decision on both screen size and aspect ratio much easier.
Also something I noticed when moving from a 56” rear projection HDTV on a 12’ x 8’ wall to a 134” screen on a 13.5’ x 10’ wall is that the grey bars in any position aren’t as distracting on the larger screen because even with complete blackout it still lights up the room like a lighthouse so light reflected off the ceiling and walls is much more noticeable than on the dark parts of the screen. Plus with a screen that fills a huge part of your field of vision the image is both big and bright enough to wash out most of the grey bars. I haven’t even bothered with a masking system because of this which is significant since I actually made masking for my HDTV because the grey bars over and under the image were so distracting in the otherwise pitch black room in my apartment. Note that this can actually create a valid argument for going with a smaller screen than the max your screen wall can fit if total blackout and total immersion were your goal.
