This chart is from Nordstrom and shows where different lengths fall on an average person. This is only a reference the actual size that fits you may be different depending on your build, thin, average or heavy, your height and also the size of you neck.
I for instance I am about average in size, but I am short and I have a thick neck. My neck measures 15 3/4 inches. The necklace I am wearing in the below picture measures 18 inches!
The below picture with the Peach Coral Measures 20 Inches.
I personally feel more comfortable with the 20 inch necklace. And yes it is because I have a double chin and I am over 50 years old. LOL. But that is the greatest thing with jewelry. It comes in all sizes and it is up to you what feels the best on you.