Best Ratios for a Newly Constructed Listening Room

14 X 20 would be nearly ideal (best to avoid square) with the speakers placed along the longer wall about 12 feet apart with the primary listening position directly across (these proportions and dimensions can be played with depending on speakers, furnishing and placement of course).  The placement of the speakers should be biased to one side of the long wall and not centered within the 20 foot side.  In other words, you will end up with one speaker roughly in one corner and the distance between them and the listener should approximate an equilateral triangle as much as possible.

Greg 

Actually Reuben - It just occurred to me after my last post that you might be setting a multi-channel theater system versus 2-channel stereo only for music.  If that's the case then my advice above probably won't work if you're doing a 5.1, 7.1, 9.1, etc.  If the latter is the case, you're going to want to orient the listening room lengthwise so you have room for your side and rear surrounds with your listening position roughly in the center of the room.

Greg

Thanks Greg, but I thought the last thing you'd want to do would be to offset the distance between the speakers and the adjacent walls? Ref. "Cardas listening room"...

Oh and this is for a dedicated 2.1 listening room.

Golden ratio works. So 12 by 20 would be ideal!!!

Congrats

there are lots of other concerns, so some general searching might be best

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.

Tens of millions of $$ are spent computing/engineering/constructing concert halls — and they're awful. They tried for 50 years to fix Lincoln Center, and it was never even close to Carnegie Hall (well, about 12 blocks). The best sounding room I ever had made no sense whatsoever — totally wacky.