When is a Listening Room Too Big

I've always considered the interaction of your choosen speaker and the size/type of listening room to be step one in getting the best sound possible. A speaker too big for your room will overload everything and ruin the sound, a small speaker in a really large room might only work well with nearfield listening.

Here's my question; when does a listening room become too large? Lets say you have a nice speaker like a Magnepan 20.7; my current room is 17.5 w x 26 L x 9 h. As I design and build my next dedicated listening room, what dimensions should I aim for? Is 21 w x 31 L x 10 h too big?

Paul Klipsch always said that the best measured rooms typically fall in a range where the width is around 67% of the length...