From smallest to largest they are: J, K, L, N, X.
Once upon a long time ago I think there was an I which was kinda J size as well but it's long since gone.
The J is what the original rimfire and .32s were made on and it then got expanded use in .38s and now even in .357 magnum. It's small and designed for easy concealment.
The K is the next step up and was designed to be a lighter and easier to carry gun for cops than the original .357 magnums on the larger and heavier N frame as it was too big for the hands of some folks and a real burden on the hip.
The L came along later when sustained use of heavy hot .357 magnum loads began to take a toll on the K frame guns. It's intended to be not much more bulky or heavy yet is stronger and more able to take sustained use of hot loads.
The N is the original .357 Magnum size and was what had been used for the .44 special and .45s prior to the introduction of the .357 magnum and was what the first .44 magnums were built on.
The newest is the X made necessary when S&W wanted to outdo the rest with the .500 and .460 S&W magnums.