rear sway bar?

Its hard to make a general rule as far as what diameter is best because most of the aftermarket sway bars have differing geometry which affects the way the bar performs. About the only way you are going to find out what works best for you is to drive the car and test different sway bars and diameters. This is what we do when developing our kits. Sway bar diameters are determined by ride and drive,slalom test, etc to determine the best diameter for the application. Using a matched set of bars from the same manufacturer will eliminate a lot of the guesswork. We also make the rear bar adjustable so that you can 'tune' the roll stiffness of the rear bar to match your driving style. .