Parin, it would be a bit better to sit a little closer than 15'(e.g., 10-12'), since there's a partial bass null directly in the center of the room. Then, if you go with a 7.1 setup(you have plenty of space behind the seats to form a rear soundfield with the back speakers), the side surrounds should be directly to the side of the listening position and the back surrounds on the back wall, separated by about as much as the distance that they're behind the seats. All the surrounds should be 2-3' above ear level.
I'll add that since you may have enough room beneath the screen, you should consider a vertical M22, M60 or even M80 for the center speaker.
