Understand Necklace Sizing
:: Necklaces traditionally come in seven lengths. Depending on the length, the necklace will hang to a different part of the wearer's body. 10- to 13-inch necklaces: Short necklaces, such as chokers, wrap close around the neck and measure 10 to 13 inches long.
:: 16- to 18-inch necklaces: Certain chains, lockets, strands, and chokers measure between 16 and 18 inches long. They stay close to the neck. Attached pendants lie above the breastbone.
:: 17- to 20-inch necklaces: Many chains, strands, and lockets, as well as some longer chokers measure between 17 and 20 inches long, meet at top of breastbone, and can attach pendants.
:: 20- to 24-inch necklaces: Chains measuring 20 to 24 inches long meet at the top of the bust line. Certain strands, chokers, and pendant necklaces also come in these lengths.
:: 28- to 34-inch necklaces: Long, 28 to 34-inch necklaces hang at or below the bust line or are worn as a doubled strand.
:: Necklaces 40 inches and longer: Often worn doubled or tripled, necklaces measuring more than 40 inches long can also hang at, or below, the navel.
:: Necklaces 48 inches and longer: Often used for long strands, necklaces measuring more than 48 inches long usually wrap or knot around the neck without a clasp. 
|Pearl Necklace |
photo by LagunaPearl
:: Add inches. To find the ideal length for a necklace that will sit just above your collarbone, use a measuring tape to measure your neck and add 2 to 4 inches. This will help ensure a looser, more comfortable fit.
:: Measure for a choker. If you want a choker, choose the standard size, which is 16 inches. For a more snug fit, measure around your neck and go no more than an inch less, depending on your preference.
:: Standard sizes for children. For children's necklaces, 14 to 16 inches is standard.
:: Match jewelry to the occasion. The jewelry should match the occasion or attire. For women, a longer necklace should be worn with a high neck such as a turtleneck shirt. A shorter chain works well with formal attire such as a dinner dress.
:: Sizing for men. For men, the necklace should sit at the collarbone when it is worn with an open shirt collar. If worn with a pendant or medallion, the chain should be slightly longer. Men's chains generally measure between 18 inches and 20 inches in length for a necklace that will lie at the collarbone.
:: Perfect pearl necklace sizing. A formal pearl necklace shouldn't be too long or too snug; it should fit above the collarbone or just below the neckline for the best look. On the other hand, long pearl strands are a stylish option for less formal wear.
:: Go long with beads. While all necklace styles can be worn long, the beaded necklace is one of the most popular types for such a length. Long necklaces are good for petite women as the length creates long vertical lines, giving the illusion of added height. 
 Understand Necklace Sizing / eBay
 Tips on Necklace Lengths / Overstock