The outer room is 22' by 12' (so that's the max). What is the ideal dimensions (as far as room acoustics is concerned)?
There are preferred dimensions, but larger is better too. My Graphical Mode Calculator software and article show that 22' by 12' (by 8') is not terrible. A length of 17 feet gives a better mode distribution, but if this was my room I'd leave it the full length. No matter what size you choose, you'll need bass traps to even the response anyway.
should I make the walls somewhat at an angle?
I wouldn't do that as it will just make the room even smaller. Angles are useful in some situations, but not so much for a home listening room or theater.
Yep _ I'm using Whisper clips to create the room within a room.
but since I'm doing so - I have the "luxury" of creating it whichever way that I like - so if very important, I can make it a rectangle - if it's not important than I'd make it as shown.