The absolute best way to select your perfect necklace length is to take a piece of string and hold it up to you at your desired length. Lay the string flat against a measuring tape and you've found your perfect necklace!
Below are approximations to help you get started:
If you are unsure what you ring size is, the best method is to stop by a jewelry store and get a professional measurement.
You may also check out this site to help you determine your ring size.