No smaller than 1". Also consider shocks and tires when going larger on the torsion bars as that greatly affects cruising quality. It will also affect your autocrossing ability.
1" torsion bars, 1 1/8" sway bar, 318 leafs, hotchkis rear sway bar, bilstein shocks, and 17" street tires my Challenger was a great cruiser, but not a great autocrosser. I upgraded the tires and the softness of the suspension made more body roll, no change in street handling. Upgraded to 1.18" torsion bars and now ride quality started suffering. Put in hotchkis leafs and with the Bilsteins and 17" sticky tires starting to get uncomfortable. Just switched to my 15" tires for the winter storage, and the car has regained its ride quality, but now I have to drive the car MUCH less aggressively.
What I have done is reached the limit of the shocks with my rather aggressive wheel rates. I have a pair of front QA1 single adjustable (bounce and rebound adjust with each click of the knob) which should help with most of the harshness because it stems from the front with the car. The rear rates are "reasonable" for the Bilsteins. I also have a 1 1/4" front Hotchkis sway bar because with my sticky tires I still have too much lean on the autocross!
Please note: just my experience. I have not autocrossed since July and that was before adding the Hotchkis leafs. At that time I was fighting issues because I was experiencing more grip than ever before. I have been cruising the car since then, and my wife provides instant butt dyno feedback about the ride.
