I was thinking about installing a projector in my family room and since I have a cathederal ceiling, there is no chance for me to mount to a ceiling. Therefore, I would have to mount to the wall across the room (about 20' away) and I was wondering how big of an image that would project onto my wall? Any idea how to calculate this? Is 20' totally unrealistic? Are there options for mounting when you have a catherderal ceiling?