2 different size torsion bars? what is preload?

Although not right, I noticed that one of mine is in backwards. It was done 6-7 years ago and although I get a creak and pop from time to time, they adjusted level and the car bounces straight and drives straight. I need to measure them and get a pair of replacements sometime.

There is no "backwards". They can be swapped end for end. The only reason to worry about which end is which is if you want to be able to read the part number after it's installed.