Building a house

1. Double wall construction with staggered studs (one wall's studs DO NOT touch the other, greatly reduces vibration transmission.
2. Line inside of walls with sheet vinyl. 
3. Double sheet rock on inner walls.
4. Electrical outlets close to ceiling to allow for wireless speakers. The more the better.
5. take steps to make heating/cooling as quiet as possible.
All I've got...

In case it wasn't obvious, the electrical outlets close to ceiling are for home-theater/surround-sound wireless speakers.

Room dimensions?
Two options:
1. Golden Ratio - Used in architecture
2. Fibonacci numbers - Used in Nature
Google them...

It dawned on me that if you are setting up a permanent home-theater (i.e. will always be in same location), you should probably put some electrical outlets in the floor where your seating will be. Modern HT chairs have USB outlets, motors and regular outlets built in. and you don't want wires to trip over if you plug them into wall outlets.

When building over a garage, make sure to tile the floor (thickest tile possible). My friend's master bedroom, converted to a listening room, was over the garage and had absolutely NO BASS. The plywood subfloor soaked it up like a sponge.