Its interesting how that ratio keeps popping up. One rule of thumb that has stuck to me is that for rooms with 8' ceilings the length should be 1.39 times the width. Aurelex goes further and suggests the ideal acoustical space would be roughly 21 x 15 x 8.

http://www.acoustics101.com/common/table5_1.jpg

http://www.acoustics101.com/room_dimensions.asp

John