I have read for good accoustics don't have room dimensions that are wholly divisible by the same number. IE 12x24 or 15 x25Ideally staying away from a square or a rectangle that is (now let me see if I say this right) 1/3 as wide as it is long. And avoid high vaulted ceilings as well.
A good number is about 15' wide by around 26' long give or take a few feet. There really is no magic number. Just keep the seating at least 3 ' away from walls particularly the back wall.
Do you have the space to go with two rows of seating?