I find that in our project tracking room geometry is most often driven (or dictated) by the host building and "what's left over" after we fit in all the other rooms, circulation, control room front walls, etc..
I know, I know...this sound kinda half-assed like we don't care about the tracking space as much.. but the fact of the matter is that you can make good sounding tracking spaces out of most any shape (other than concave arcs) if you are a little clever about it. We study the available space, the architecture of the rest of the project, the building, etc and try to make a design for the larger tracking spaces that compliments the building (when in a free standing structure) the rest of the studio spaces and sound great! This is an interesting and very intuitive phase of our designs, so I can't give you any magic advice... the things we look for include:
1. Proper amount of reflective, diffuse and absorptive surfaces based on the size of the room and the goal of the design (live room, V/O, foley, etc...)
2. Creative use of surfaces... we dont want to do the same thing over and over an over and over.... ya know?
3. Site Lines .. from the CR to all recording areas and as from all booths to as much of the Live room as possible.
4. Avoid flutter issues (self expanatory)
5. What type of floor finishes do we and/or the client want to use..
6. How does this affect our design direction for the ceilings (which in turn will affect how much of what types of surfaces we will use on the walls, etc...).
You see everything affects everything else, so it usually develops over a few iterations.....
BUT, there is no set geometry or shape that we look to create for live rooms... Large rectangular spaces are the easiest to start with , I guess??