Some other questions are.
1) What tow vehicle do you have?
2) How big of a guy are you?
3) Do you want to stay compact or would you rather spread everything out for your work area?
4) Is money a concern?
I have a 6 x 12 that has 2 bunkbeds across the back and 2 of us can pit in it over a race weekend comfortably and run at least two classes. But then we are both under 6 tall and about 150lbs each.