For me I went 7.1 (will be 7.2 if I ever build my second sub - have all of the electronics for it), but that was really mainly because I had 2 rows of seats and I wanted the larger surround sound-field to envelop both rows of seats. If I didn't have a 2nd row I would probably have just stuck with 5.1 (or 5.2).

For reference, my room is 14' wide, 24' deep (but with a 2 foot deep false wall up front) and 7'9" tall.
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M60s, VP180, VP150, QS8s, M3s, SVS 20-39PCi, Shakers