Please don't cross post between different forums with the same question.
You had asked the question here:http://www.bigscreenforums.com/forum_topic.cfm?which=16662
Since buildings can offer various levels of reflectivity and gain structure, and there is no indication as to the ambient conditions, it's nearly impossible to say how things will look or what the results will be. Since the size is likely to be in the same realm as commercial movie theater screen sizes, you should remember that those projectors run upwards of $80,000 to fill that screen in a 100% dark environment.
A cheap, non-theater grade model will likely be in the $15,000-$30,000 range to do the size you are talking about. Then, you will still have a lot of unknowns to deal with.
So, I will stick with my strong recommendation that a high-end rental projector is recommended prior to any purchase.
- Theater, whole house audio, and technology consultation during the build and installation process in the Washington, DC metropolitan area.