The most important criteria is that there should be no major mode of resonance greater than the speaker -3db rolloff.
To calculate the lowest major node (below this frequency the volume falls off rapidly):
DIVIDE 172 BY THE LONGEST LENGTH (IN M) TO GIVE A VALUE IN HZ.
The speaker bass should at least start to roll off above this frequency or you will be introducing peaks and booms due to the room reinforcing the response of the speaker. There are of course many other considerations, bu tthis is a good starter.
Great question! Too big to really get into here. From an acoustical point of view, soundstage is affected by so many factors. I would say the primary is horizontal symmetry.
For a good soundstage, the room should be at least 15' wide. Less than this and you receive too much boundary interference to deal with, and/or your speakers become too close to each other to offer soundstage depth. For a good soundstage, you should absorb and/or diffuse all first order reflections (by about 15 DB SPL below the direct) down to at least 500 Hz.
For or good bass response, the room should be rectangular in shape and offer good room mode distribution. In addition, the speaker/listener positions must be such that they do not exasperate the existing modes and still offer optimum stereo imaging. Also, for articulate bass response, the shell design should include damping for low frequency absorption and cavity resonance control.
For good resolution and dynamics, the room shell systems should include breaks, absorption, isolation and/or blocking of noise interference. HVAC is obviously part of the overall noise control issues that must be addressed. Electrical is also part of the noise floor regarding dynamics and resolution, not to mention many aspects of sound quality.
Calibration. Without physical and electrical symmetry you can't have an optimum soundstage.
Additional interior acoustic treatments are also needed to further control room modes and reverberation times so that the room sounds neutral.
This is our performance value hierarchy for audio systems:
'For a good soundstage, the room should be at least 15' wide'
Yes I mostly agree with this. There is actually a trick you can use for narrow rooms:
Use speakers that don't go down enough not to excite the room width - probably best placed on the longer wall to give any decent separation. Place a calibrated subwoofer 1/3 to half way along the width, next to the wall. This will then excite the length without exciting the width too much.