There are different standard necklace sizes for women and men as below:
First:women’s standard necklace sizes:

14″ NECKLACE
Fits most like a choker

16″ NECKLACE
Falls perfectly around the base of the neck like a collar.On a petite woman it will hit your collarbone while for plus size women it might feel more like a choker.

18″ NECKLACE
A common choice for women that will fall just below the throat at the collarbone. This length is popular for adding pendants which will hang over a crew neck.

20″ NECKLACE
Will fall a few inches below the collarbone and allow a little more breathing room. Perfect for a low, plunging neckline or if you want to wear it over a turtleneck.

22″ NECKLACE
Falls at or above a low neckline.

24″ NECKLACE
Falls below any neckline.

36″ NECKLACE
Common with pearl strands and will hang below the bosom. They can be worn as a single strand or you can double them for an 18 inch necklace.

Second,men’s standard necklace sizes:

18″ NECKLACE
A shorter length that falls just around the base of the neck.

Popular for beaded necklaces so they can sit above a crew neck t-shirt.
20″ NECKLACE
A common length for men which falls right at the collarbone.Pendants will hide under a crew neck or show completely with a low v-neck or unbuttoned dress shirt.

22″ NECKLACE
Falls a few inches below a collar bone and can be worn over or under a shirt. This is a common length for chains because they offer more breathing room.

24″ NECKLACE
Extra long and is commonly used to keep religious or meaningful pendants under the shirt and closer to the heart. Falls just above the sternum.

*Please keep in mind that these are all approximations for an average woman’s or man’s height and size. Larger people will need longer lengths and petite ones will need shorter lengths.