Picking the right size necklace is like sizing any other piece of jewelry; you'll need measurements. It's recommended you use a flexible measuring tape to measure the circumference of the wearer's neck. You can then consider the type of necklace you're looking for so you can add length if needed.
For example, the precise measurement of your neck is great for chokers but isn't ideal for pendant necklaces. You'll want to consider adding another two inches to your measurement so the pendant can dangle effortlessly. If you want an even longer chain, consider adding four to five inches to the total necklace length.