What's the 'ideal' room size?

This is actually a complex subject. You could read the Master Handbook of Acoustics and use the required room optimizer software and then hope you get it right or you could hire Richard Rives who is an experienced acoustic engineer specializing in home audio. When and if I build a new home, that is what I will do. Your room has a lot to do with the quality of the sound. My recollection of the handbook is that it gave a range of desirable ratios and suggested a minimum volume.

Wow Ozfly - nice post and I think it's exactly what Eandeely needed. Room acoustics as you point out are so important. They however get way too little attention. In my case, I know some RPG Skylines on the ceiling would help my system a lot. Unfortunately, my wife steadfastly refuses to let me get them. She insists they are too ugly for our living room. But darling I say- whats worse ugly sound or ugly looks. She says looks. See the problem.
By the way- the pictures in the Master Handbook of Acoustics of various acoustically treated rooms and studios are way cool and inspiring.