Projector and projection screen questions?

I want to get a projection screen and projector for my room, to watch movies, play games...etc. My room is a pretty big room but not huge, I don't know the exact measurements but typically how do I figure out the max size projection screen I can get, and what projector I can get. I read that the size depends on the distance, but then I also read that you can change the setting of the projector to make a bigger/smaller image. My projector will be about 15 feet from my screen, will this work? Also, does the projector have to be high up? Because I am wanting to connect a Nintendo 64 to it, and since the controllers aren't wireless, It can't be very high up. Thanks and any help is appreciated.