brownsfan made many good points. I suggest that you find a good on-line Room Mode Calculator which speaks of Schroeder Frequency,
Bolt Area, Bonello, etc. to help you better understand the consequences of room dimensions. You might consider consulting with a professional because once the walls are up you are stuck with the design.
You might also consider non-parallel walls. Our purpose built room was designed with non-parallel walls and the results are well worth the effort. The calculations are more complicated but worth it. They help minimize the need for absorption and diffusion. The end result is a lively (not bright) well balanced room. The next time you are at an indoor live venue check out the enclosure you are sitting in.
If possible, consider which family of speaker design (mono-pole, dipole, point source, line source, omni-directional) you will likely end up with. This consideration doesn't necessarily influence the optimal room dimensions which is more about distributed room modes and a smooth bass response but it will influence the interior room design and treatment needed.
Consider built in or free standing corner bass traps. There are real reasons why qualified professionals recommend corner and soffit bass traps. Don't believe the general statement that too many bass traps will absorb all the mids and highs and make a room sound over damped. Not all bass traps are created equal. For example, DIY "bass traps" that are basically a 4" thick block of Corning fiberglass wrapped in fabric are marginally effective as bass traps and are really broad band absorbers that can, if overly used, over dampen a room. A REAL bass trap (bass attenuation only) typically has a membrane covering the fiberglass which reflects the mids and highs back into the room and does not over dampen the room.
