The first two comments give good advice, but even better, I think, is this: get F. Alton Everest's Sound Studio Construction on a Budget and his Master Handbook of Acoustics. The latter sounds better but the former is much newer, and better on room-treatment devices. Both tell you how to analysis your room for main resonant nodes, and Master Handbook has a particular suggestion for dimensions. But don't do what I did with a dedicated room: put in a sloping wall-ceiling (cathedral ceiling). Hard to work with! If you want advice on homemade RPG-style diffusors, get in touch with me--but you'll need a table saw to make them. Good luck!