I thought the length couldnt be changed? I've not come across one that's legal to unscrew a 2" piece and shorten it. But interesting if I'm wrong.
How much they reduce noise is a function of volume. So longer should be better all other things being equal. But bigger diameter increases volume much more than adding length. A 1.5" can 6" long has volume 10.6 cubic inches, increased to 10" volume is 17.7cubic inches. Bump it up to 2" diameter at 6" and you get 18.8 cubic inches. But you dont see many large diameter cans out there, they would get in the way of optics, and they probably just look goofy.
Edited for clarity