If the overhang is 9 feet high then you'll want to consider how high up off the ground you want the screen to be. My guess is you'd want about 2 feet of clearance off the ground, leaving you with 7 feet for your screen height. If you want a standard 16x9 HDTV-aspect ratio screen, then it would be 12.5 feet wide by 7 feet tall.
As for what material you use, I agree with cecilb there are a lot of options for you to research. The material you choose depends on many factors, like whether you're going to leave it outside all the time. The best place to start is in this thread, section 3, "Screens":http://backyardtheater.com/forums/index.php?topic=284.0