The above photo is a general guide when selecting a chain length. The 18 inch length is a standard size. If you like to wear your necklaces longer (or are very tall or plus size), a 20 inch or 24 inch is recommended. The 16 inch length is too short for most people and is recommended only for petite, slim builds or teens.
If you are unsure of the chain size that is right for you, please make sure to measure a chain that you like or use a string to simulate a necklace and then measure that string.