Smallest room dimensions for good bass to 20hz

Arthur is right - big rooms need lots of powerful bass reproduction - very expensive as loud accurate bass is exponentially expensive. So the sound is indeed far better in a large room provided you have the resources to fill it with sound at necessary high SPL.

The one downside of a large room is long reverb times in the bass...you need plenty of LF absorption on surfaces and soft furnishings to keep this down (wood walls help leak out bass whilst concrete can be bad)....think of how lecture hall or brick gymn sounds awful(this is obviously far to big and too reflective)

My guess is that around 25 to 35 foot dimensions make a good large room with significant improvement over 15 feet. After this improvements diminish and eventually too large is bad....some wall reflections are actually pleasant remember that speakers outside in the open air sound hollow.

Although 20Hz is 56.6 ft long, one needn't 'fit' the whole length in the room, just 1/2 the wave will do nicely, which gives 28.3 ft. Also, the corner to corner (ceiling to floor)distances must be factored in and of course those dimensions are always longer than any one of the three other dimensions.

Aball, I guess more information would help. I have fairly large speakers, Aerial Acoustics model 9s driven by a Parasound Halo A21 amp at 250 watts per channel. At present they are in a L shaped room 7ft high, about 25 feet long on both ends, and 14ft wide. These speakers go down to 30 hz and in this room don't overload the room. In the future I will build a dedicated room for these and don't want to build to small a room for this set up. I wont have the room to build a room thats bigger than 10ft high 25ft long and 16ft wide. I also don't want to build any bigger than need be, but have heard that full range speakers can overload a small room. I also may add a good sub or two to fill out the last octave.

Upper corner front left to lower corner rear right need to be 1/2 of 56.6 ft. That length works out to be the SQUARE ROOT of:
[(square root of: L squared + W squared) SQUARED + H squared]

Of course if your listening room has at least one wall which has (roughly) half its total surface area open to an adjoining room, you need to measure the longest top-to-bottom diagonal of the combined spaces ;-)) Have fun!

If you are going to the expense of building a dedicated room, why not get an acoustical consultant like Rives in to help you out?