14" - wraps closely around the neck; also a standard size for children's necklaces
16" - draws attention to the neck; a pendant might fit right at your collarbone
18" - is the most common length; hangs just at the collarbone
20-22" - drapes low around the neckline; a good choice for casual dressing
24" - is the most dramatic; often worn with a large pendant for a striking statement piece
28-36" - can slip over the head and wrap around the neck twice to form a double strand