I don't know of a rule, other than "does it look okay?"
Classic fountain pens (such as Parker, Sheaffer, Waterman, Conklin, Esterbrook ...) have often used clips that are shorter than I would expect yet they still look good. I've made a few clips that were longer and/or fatter than looked right when I got done - easy enough to take off and put one that has a better match, or grind it down a bit.