Torsion Bars... Best Diameter for a Lowered Street / Autocross car?

I got a book on how to setup A bodies for autocross, gives you formulas need find center of gravity and roll centers etc.. But still it only gonna give you ballpark and if you Plan on pushing car and tires to the limit your gonna have to play with different bars and springs and shocks tires etc... like AndyF said.
Generally you want the rear the give slightly before the front, but at top level that can very from course to course or even corner to corner, everything is a series of compromises. If your not pushing to the limit as long your not bottoming out and keep the car somewhat flat through the corners it's probably fine.