Props to Jmcgrogan2 for providing you with a very useful tool.
Here's some rule of thumb guidelines.
Given a 10' high front wall, you need to allow at least 4' on the bottom (to clear heads) and a foot on the top (to avoid bounce off the ceiling). So effectively you can have a 5' high screen.
Assuming you do a 16:9 screen, you would have a 106" wide screen - about 8'8". If you look at the handy dandy calculator you will see that this is a bit big - plug in values but somewhere around 80" wide is a lot more comfortable. In this case your screen would be 45" high. (Note these are net screen dimensions without allowing for frames, mullions or other trim)
Not sure what you mean about not wanting to place the projector about the seats. It's pretty much a given that you are going to have to go for a ceiling mount. Make sure you take wiring (especially AC) into account as projectors are very sensitive to power - a dedicated circuit and a a conditioner would be many well spent. Worth comparing projector noise stats as well.
Projector distance (what we call throw) is a function of the lens. Here is a handy dandy calculator for that:
http://www.projectorcentral.com/Epson-PowerLite_Home_Cinema_1080-projection-calculator-pro.htm
Realistically you want a projector with a zoom lens with a keystone adjustment function to allow you to compensate for the lens not being at the center of the screen (ideally the center of the lens and the center of the screen should be parallel and perpendicular to each other)
Finally there is a trade-off between how bright the projector is (lumens or foot candles) and how bright (reflective) the screen is which is expressed as gain.
The goals of course is to have a bright even picture without glare. Problem is that the higher the gain, the narrower the viewing angle (meaning that the image gets darker/dimmer as you move off center to the sides). Given that you are intending to use the full width of the room as well as the length, it would seem that the optimum balance would be a lower gain screen and a higher output projector.
The friendly people doing your installation, or the people at Stewart Film Screens can advise you about this in greater detail.
BTW You probably won't want to seat back to 11' since it will make it difficult to adjust the delay on the surrounds effectively.
