Well, your area (room size and dimensions) is sufficient for a 7.1 setup, but the window/wall locations and placement of your couch isn't. If the couch were 3 or 4 feet closer to the display, leaving some space behind it for rear mounted surrounds, and if there were reasonable locations for side surrounds on side walls, you certainly have the space for 7.1.
But, in the real world, in multipurpose rooms, we all compromises a bit when it comes to speaker placement. It's just the nature of the beast. If 5.1, with the surrounds spaced out on the back wall, is the best you can do, then set everything up, sit back, relax, and enjoy the movies and music.
