Should I be concerned with speaker placement or should I go with a larger screen?
This has all got me considering 16/9 again.
My personal preference leans towards video, so the larger the screen the better and scope screen definitely. As long as your intended pj has the horsepower to light the screen to satisfactory levels and still provide black levels suitable to your tastes and viewing. I've placed my Onix Rocket 1000 towers closer to the walls to get a 120" wide scope screen in a 14ft wide room. I have an avr with audyssey xt32 that handled the set up very nicely.
My eyes are 13.5 ft from the screen, I mask the scope down to 91 x 51 for hdtv. This is in a bat cave and my screen is only 1.1 which musters 14ft after calibration on my Sony vw95. The size for both hdtv and scope movies is most satisfying and of course more is always better, right??!!. I've been tempted to bump up the screen width to 136" but that would only leave 16 inches on either side of the screen for the 51' tower speakers which I relented and figured they would be too close to the side walls even toed in as I have them now. I could easily move them there now and recalibrate the speakers to see how they would sound but moreso I'm concerned about the drop off in brightness which it seems when moving up to screens larger than 10-12 feet wide that only the uber expensive DLPs and the new Sony 4K can light up to get that 'pop' and still have great black levels, at least for non high gain screens.
To answer your question if I feel it is big enough...I do...but if I could get a pj that I could maintain a 1-1.3 gain screen and provide even higher brightness with reference black levels at price in the sub 15K range I'd probably get the larger screen provided the tower speakers sounded reasonably balanced closer to the side walls.