Many of the great concert halls are sized for the music, sound better with lots of people in the seats and actually have a lot of acoustic treatment -- sometimes the treatment is more obvious than other times. Often overlooked architectural offsets, uneven walls, curtains, etc. all contribute to the nice acoustics in these environments. Sometimes, the acoustic treatments are much more obvious and include large diffusers in the rear and side walls and ceiling absorbers.
In any event, size is a good place to start with a dedicated listening room. A 20Hz wave needs about 28 feet to accommodate half of it so, if possible, that's a good place to start. Reverb time is a function of the cubic area of a room, the surface area of the room and the absorbtion coefficients of the surfaces -- again, big rooms are good. Both reverb time and the nature of standing waves and reinforced nodes (i.e., where each of the room dimensions "support" or don't "support" certain frequencies) in, especially, smaller rooms contribute to the quality of sound. Generally, the treatments most audiophiles actively pursue are necessary because the rooms can't be designed toward an ideal from the perspective of size and surfaces. There is no perfect room dimension or character, but the great concert halls tend to follow similar sets of sizing ratios.
There are some great books (e.g., Alton Everest's books) and sites on the topic. One of the key principles have to do with getting the dimension ratios right: Some good ones are a) 1:00 to 1.28 to 1.54, b) 1:00 to 1.60 to 2.33, c) 1:00 to 1.40 to 1.90, d) 1:00 to 1.50 to 2.10 and e) 1:00 to 1.26 to 1.59. You want to avoid a cube (since all three dimensions reinforce the same frequencies) or any two dimensions being the same. Smaller rooms especially benefit from bass traps that could tone down some of the excess bass that is reinforced by the room. Other key principles have to do with reflection times (e.g., the rear wall reflects music back to you after you hear it from the speaker itself) and reverb times (i.e., how long it takes for the room to decay the sound by 60 db). Nothing is really simple about this -- e.g., an "ideal" reverb time for rock music is different than one for opera. Room treatments can also make a big difference in these, with diffusers affecting the nature of reflections and some treatments absorbing music quicker to give a faster, flater (across frequencies) reverb time. This is especially important at points nearest the speakers where the reflections occur first.
I'm a rank amateur on acoustics but I've found it very valuable to study up a bit before building my listening room. If you're in the process of constructing one, I would encourage you to do some reading (or hire an expert) to get the most out of it. Good luck on your journey.