If you have 2 rows of seats in that space, then I would highly recommend 7.1
If you have one row, and it is towards the back of the room (a LONG distance from the screen in my opinion), then go 5.1
My room is 14 feet wide, 24 feet long, and just less than 8 feet tall. I have two rows of seats and 7.1 sound.
It doesn't seem that much better or really even noticeable unless I do an A/B comparison between 5.1 and 7.1 while listening to the same material. However, it is critical when sitting in the back row as the "wide surrounds" are behind my front row (on the side walls) but in front of my back row. Without the 2 extra speakers/channels in the back, the sound would never envelop the rear seats.
M60s, VP180, VP150, QS8s, SVS 20-39PCi, HTPC, JVC RS45, Onkyo TX-NR709, Shakers