You can check out the throw distance for your projector at projectorcentral.com. Many good projectors work with an 18' throw distance on a 10' wide screen. You could put the projector in the garage and fire through a port hole if you wanted to avoid noise.
If you go with a 10 foot screen you will probably want to do an acoustically transparent screen with speakers behind the screen. Otherwise your speakers are going to be crammed in the corners.