Most people aren't really sure what length necklace they would like. The following chart gives an indication of necklace lengths - be aware that where the necklace sits will differ a little if you are bigger or smaller than the average person.
14" is a choker and should sit firmly around your neck.
If you would like your pendant to sit in the little hollow of your throat, 16" is the length you are after.
A good average choice is 18" - it's generally a comfortable length for most people, and it sits above the neckline of your clothes.
Slightly longer is 20". It's a lovely length for a string of pearls, but not so great with a pendant as the pendant can tend to disappear into the top of your cleavage.