The stock 356 A and B cars had a 24mm rear bar, and the stock C cars had a 23mm rear bar with the same rating as the A/B bars since they were shorter.
Going up one size in the rear bar, in combination with the stock front bars, will improve cornering, especially when used in combination with the WR 17.5mm Front Swaybar Kit.
WR Rear Torsion Bar Sport Bushings are recommended.