According to Amroc a 15ft CUBE room is much better than a room with dimensions 20-35ft.-Acoustics Forum-Acoustic Fields Forum

You can not isolate one varaible in acoustics and consider it independent of others. There is no ideal room size. That is like saying there is only one flavor of ice cream. You must first look at intended room usage. What will you be doing to generate energy into the room. What instruments will be used to generate this energy. What is the frequency response of these instruments or energy sources. Do the instrument frequency responses and amplitudes (strengths) fit into the room. Which frequencies at which amplitudes fit and which ones do not. Nothing in acoustics stands alone. Those that try to simplify the process. They do not understand the physics behind sound energy in smaller rooms and secondly, this misunderstanding always produces less resolution. Improving room resolution is a forward moving process with many steps that can not be skipped or cheated around. You will always pay for such indescreations with less resolution.