I wouldn't recommend a rectangle at all. Although that's what is mostly written up--it's done so because acoustic calculations for standing waves are pretty simple, and most rooms are rectangular. If I were building a dedicated listening room there would be no parallel walls or floor to ceiling height.
If you could...
If you could build a listening {not theatre} room that you did not need to use for anything else, what dimensions would it be? Ceiling height, etc... for optimal sound quality. Assuming an above average system.