What is your budget? Is this a temporary setup? How do you plan to transport it?
I have 8ft screen for court room presentations. It's light, fits in a car and an elevator and easy to setup. 10-12ft being the maximum length I want to deal with. Anything larger becomes expensive and difficult to setup. The really large ones involve assembling a frame and stretching material across which is difficult. For this type of screen people hire a rental company and it's not cheap.