Dedicated listening room 25' L X 9' H and ? W

any dimension greater than 13' up to say 17' 6" would be a good start.

Reason, if you double H=9 you get 18 and this would create too many like room nodes canceling each other.

15.45 feet wide for a 25 foot deep room.
Use the Golden Ratio which is 0.618 = (sqrt(5)+1)/2
This is also the ratio used in the Cardas speaker placement formula.
This is a truly remarkable number with numerous websites dedicated to it:
Fibonacci

We can definitely help. As to the width, you are in the ballpark. It takes a little time to determine the exact width. I do like non-parallel walls, but you have to consider the whole room for this to work. You also have to consider that the largest problem in room modes will be from floor to ceiling as this is the largest surface area of two parallel surfaces. Splaying the walls actually doesn't get rid of room modes, but it does reduce the Q factor (sharpness of peak) and does make modal distribution better (in most cases--there are exceptions of course). As to your question about corners--no rounded corners is generally not a good idea, but building frequency dependent traps into the corners (based on the room response) is a good idea. Feel free to give us a call or look on our website www.rivesaudio.com to get a better idea of what we offer.

Talk to Rives. They are the best!