If you have the choice to create the rooms from scratch, dont make the width and length of the rooms the same. In other words, dont make the rooms squared. There is alot of science behind the reasoning but most of it can be explained here
http://www.ethanwiner.com/acoustics.html and other places on the net.
As far as the vocal booth, I am not a physics wizard but I would assume the general physics would be the same as your other rooms except on a smaller scale. I would use the same techniques as far as the layout, and make it just big enough for however many vocalists you want in there at the same time. Plus add a little room for sound controling materials. Like the control room, I wouldn't make it a perfect square.
Another thing that I would suggest (if the rooms are already built) would be creating the vocal booth using the 6x6 room you designated for the control room, because I dont think you will be able to move and/or breath inside of a 6x6 room with even the minimal gear (desk and chair). It also wouldnt sound good acoustically and would be very hard to control using any method due to its small size.
Since it sounds like your one of the lucky ones able to build from scratch I would aslo suggest doing as much research as possible on acoustics, and room design. Proper design will save you from many pitfalls that us more unfortunate have to deal with!
I am far from an expert on anything I just talked about. Most of this is just information I've put into my brain in the past few days. Though most of it, if not all of it should be accurate enough to start with. (i hope?)
There is good overall insight into spacial acoustic purpose and treatment.