I think generally speaking, bigger is better. You want to stay away from dimensions that are equal, ie length != width != height. Cubes are terrible, depending on the size, certain frequencies are reinforced and cancelled, you end up with an acoustic nightmare.. I think dimensions that have low common denominators (like 8x12x16, 10x15x20, etc) are bad for the same reason, altho not as bad as cubes (where the lcd=1).
I'm not a pro, and I'd have to do some research to make sure, but this is my initial thought: If I was building a custom live-room, I'd probably go with a minimum of something like 8x13x19 (assuming 8' ceilings) - and I'd vault the ceiling if possible.