Best Ratios for a Newly Constructed Listening Room

Well, certainly don't make the room square.  Before you do anything you should read Premium Home Theater by Earl Geddes.  No longer available in print but can be downloaded.  You don't need to be an engineer to read it, and it will give you a good foundation of room building fundamentals with practical instructions and recommendations for materials to use.  You'll be glad you read this.  Best of luck. 

Here's a link...
http://www.gedlee.com/Books/Books.aspx

I would also recommend the Cardas.dimensions. Cable Company uses the Cardas formula in all of their listening rooms. Every time I go there, I'm always impressed with how good their rooms sound.

when i built my house and my music room i researched this... the dimensions are as you would prefer, depending on your use for 2 channel or more, HT or just audio etc etc

main takeaway is the avoid square dimensions and any dimension that is a clean mutliple of another like 12 x 12 x 9... that is a recipe for standing waves/nasty resonances

i went with 19 x 17 x 10 ft ceiling

Don't forget to take into account your speakers, as different models may require specific placement.
Kenny

Best thing may be a sloping ceiling.  There are LOTS of design issues that should be addressed.  You can start with:

Master handbook of acoustics / F. Alton Everest ; Ken C. Pohlmann.

try to get the latest ed.