Another problem with larger torsion bars is that the chassis may not be able to handle the larger bars. Essentially the unibody becomes a spring. If subframe connectors, torque boxes, etc have been added, larger bars become more effective.
Also, larger bars can allow for lowering without bottoming. The larger the bar, the lower you can run the car.
The final factor is tires. 15" tires have more give than 17" as the tire is also acting like a spring on the surface of the road.
My Challenger used to run 1" torsion bars and Bilstein shocks, and I hated the ride. Hindsight being 20/20 I think I had it lowered too much.
Now I have 1.18" bars and QA1 single adjustable shocks. Its a very firm ride and with 15" wheels but not unpleasant (but very easy to overpower the tires). It works well for autocross with 17" wheels.
Torsion bars and shocks are part of a complete package. 1.03 like AndyF suggested is a good all around diameter. But the caveat is, over the counter parts house shocks, won't work with them, either.