'Nook, welcome. As some of the replies indicated, the determining factor isn't the size of the room, but the availability of enough space behind the listening position for a good rear sound field to develop. There's no hard and fast rule, but at least 4' is a common suggestion. If not, a 7.1 setup wouldn't likely be of significant benefit(and a single rear surround in 6.1 wouldn't be either), and mounting the side surrounds a couple feet farther back than directly to the side would give a bit of phantom back effect to help out some. You could consider moving the couch slightly toward the front to provide more space behind you.
