This is a complicated issue, there being no one "right" size. See F. Alton Everest's Master Handbook of Acoustics, 4th edition, or his Sound Studio Construction on a Budget.
Several versions of the "ideal" are given in one or the other of these books. Not just proportions count: the bigger the room, the better its resonant modes will be spaced, if the proportions are good. Reading Everest will teach you how to analyze a room yourself, and see how good it is and where its problems lie.
Several versions of the "ideal" are given in one or the other of these books. Not just proportions count: the bigger the room, the better its resonant modes will be spaced, if the proportions are good. Reading Everest will teach you how to analyze a room yourself, and see how good it is and where its problems lie.