My main tracking room is 13' X27' with 9' ceiling. This works well for 3 or 4 piece bands, more than that and it starts getting crowded, most musicians seem to preferr having some space, and too much gear or too many people gets cluttered very quickly. Rooms of this shape tend to be "boomy" sounding and have some reflection problems. This is where room treatment really pays off. Diffusor panels (for echo/reflections) and bass traps (for the boominess) are more than worth the cost and trouble of installing them. Lighting is something else to consider. Avoid the use of florescent lights, they tend to cause a lot of hum, some dimmer switches cause hum too.
You mentioned having a 2 room set up, is the second room going to be used as a control room or as an isolation room? Either way you will need to plan for some treatment there also. Window between the two rooms? Glass presents it's own problems.
I commend you for planning in advance rather than building a room then wondering why it doesn't sound good. Take your time and build it right and you will have something you can be proud of.