ideal room size

Go to the Cardas website (www.cardas.com I think).

This is the room dimension ratio (from memory), from Harley's book.

1.0(Height) x (1.9)Length x (1.4)Width

This is a complicated issue, there being no one "right" size. See F. Alton Everest's Master Handbook of Acoustics, 4th edition, or his Sound Studio Construction on a Budget.
Several versions of the "ideal" are given in one or the other of these books. Not just proportions count: the bigger the room, the better its resonant modes will be spaced, if the proportions are good. Reading Everest will teach you how to analyze a room yourself, and see how good it is and where its problems lie.

I have the book and have gone to various web models, but none of them talk about how to deal with the calculation for stadium seating. My 32'x15'8.5' room has 3 levels. At 15' it goes up 8" and at 21' it goes up another 8". I'd love to hear from someone that has a model that takes that geometry into consideration.