There are books that have proportions for jewelery and faces. I'm interested in this sort of thing and I just ordered one called Jewelry Savvy. You can preview pages in the book and some of the ones available to read are on earrings. I looked up the book on Amazon and then clicked "search inside another edition of this book" -- if you persist, you'll be able to read a little bit about what earrings are right for what ear shape, face shape etc... and see if its a book that might help you.
I need to check this out. I have a narrow face and I don't want to wear earrings that emphasize that.