Actually, they should be close to being identical in length and look similar to each other. If another fish bit the fin off, it's understandable that they are different. If they are different from birth, then it's a genetic flaw and an undesirable trait to most Angelfish officianados.
Neither is a problem to the fish's health so don;t worry about that. You can go to the AngelfishSociety.com and see what a "normal" Angelfish should look like.
Hope this helps