Sizing
This page should help you gauge what size of jewelry will fit you best. I will show you a series of pictures where I am wearing different length necklaces.
Another handy way to figure out what size of necklace you would like is to cut a piece of string to a certain length that looks good on you. Measure the string and you will have a pretty good idea of which necklaces would fit you correctly. You should also take into consideration that if you are purchasing a necklace with a large pendant, that will hang down a lower on your neck. You can also do this measurement test for bracelets and anklets.
My neck is approximately 13 inches around and I typically like to wear 16 to 17 inch necklaces, these do not fit like chokers on me.

This necklace is 14.5 inches long, the pendant pulls it down a bit making it look longer.
This necklace is 16 inches long, notice how the pendant above makes the necklace seem longer.
This necklace is 17 inches long and hangs slightly lower down.
This necklace is 18 inches long.
