Sir, as Randy pointed out, the main factor isn't the size of the room, but whether there's enough room behind the listening position for a back surround effect to be experienced. There should be at least 3-4' for this, so your front row should benefit significantly from the back surrounds. My suggestion would be to do the 7.1 and put guests who you don't especially like in the back row.

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Enjoy the music, not the equipment.