Chadnliz, pay attention to Jkalman's advice. If you shorten the 40' dimension by 8', you end up with 32', depending on exact wall placement. Then you would have dimensions of 32 and 24, all divisible by 2, 4, and 8. This would not be good for mode distribution. If this leaves you confused, suggest you search archives on AudiogoN and Audio Asylum. Lots of information is available on this subject.
Potential new room, will this work well?
My parents are house hunting and I found a great place with a connected second building that is 40 ft by 24 ft with 11 ft ceilings......speakers will almost certainly be Vandersteen 5A. The room may be made 8 feet smaller to add a small second room for tools wich will extend across entire short wall. Thanks