Optimum size for music room?

I'd say 23'X37'X9' should give you enough room to play with.

If you search the internet for "golden cuboid" you will find many discussions about the "perfect room". Note especially the Cardas site as well as many others. I have never had the opportunity to build a dedicated room so my equipment had to adjust to the room dimensions. In my mind the most important criteria would be non-parallel walls, enough distance from the front wall for your speakers to image well, acoustical treatment for reflections, dedicated power, and sound-proofing from extraneous noises. I hope you get to build it. Let us know how it works out. A real "man cave"??

There is a relationship called the Fibonacci Series. Cardas says it is to be found everywhere in nature including the nautilus (snail?) they use in their logo.
Essentially it provides the relationship between length, width and height which provides the least reverberation and cancellation.
I imagine there are other people on this forum who can explain it better than I can but I believe I calculated something like
22 x 13 x 8 once upon a time.
Draw a line about a foot long running from left to right. On the left end write "A" and on the right side write "C". Then you figure where to place "B" along the line by locating the point where BC is to AB as AB is to AC. In other words, find the location where the shorter segment is to the longer segment is the same as the longer segment is to the whole line.
As it was explained to me, this will be the ideal dimensional ratio to employ in designing your room.
However, I like the idea of non-parallel walls too, and I cannot imagine how that gels with what I just proffered.
Good luck. This doesn't seem like a good place to proceed by trial and error.