When planning a music room for our home that was built in 2002,
I had posed this question on the theatre organ bulletin board.  The
one guideline I chose to follow was that a rectangular room was best.

A formula was followed that the sum of the ceiling height (maximum)
plus the length of the short wall should equal the length of the long
wall.  In our case, we ended up with a room some 36 feet by 50 feet,
with a maximum ceiling height of 18 feet.  Not quite to the formula,
but close, as we added some square feet during the later stages of the
planning which threw off the formula  somewhat.

In any case, the ceiling should not be flat.  Our builder used "scissors
trusses" which give the ceiling a peak down the long middle.  For our
Allen theater organ, the room turned our really well with a built-in
reverb time of 0.6 seconds.  Chairs and other instruments in the room
help with echoes.  That is my two cents.

Dick Leis in Florida