The magic number for the dimensions of acoustic rooms is Phi, 1.618.
1.618 : 1 is known as the golden ratio. 1.6 is close enough, so, .62 : 1 : 1.6 : 2.6.
So, about 5m x 8m x 3m is ok.
The idea is to have the resonant frequencies of the parallel walls tuned differently to avoid harmonic relationships.... the ultimate acoustic disaster is inside a cubical room, where all 3 dimensions resonate at the same frequency. It rings like a bell.
Non parallel walls would be preferable, but builders don’t like that, or carpet layers etc.. but a slop