Listening Room Dimensions... I'm building one and need your help

Sounds simple but I'm having a hard time coming up with most 'ideal' dimensions for this dual -purpose room that I'm building this year (hopefully).  



Purpose #1:
listening room for 2-ch hi-fi 
I have BIG speakers- several pairs 
*ideal is approx 25'x40'x12'
*want to sit 6-10 people in prime listening area 

Purpose #2: (other side of the room)
band rehearsal area (loud guitars, drums, keys, PA system, lots of 
*ideal is...bigger the better 

- I've read really good threads about general guidelines especially from Doctor Fine- hope he jumps in...
- I know the Cardas, and other rule ratios. But this room is a combination room so I'm not sure how much that changes anything 
- I know I'll probably have to use quite a bit of sound treatment 


My first inclination before research was 30' x 50' x 11'. 

What dimensions would you recommend and why??