Use any tape measure to measure you wrist, neck or foot.
Now, round up the measurement to the next bigger size that is available.
For example: Al's neck measurement is 13 1/4". Round it up, and it comes up to 14" choker.
For loose necklaces (i.e. not chokers)
For necklaces that are not tight, you can loosely measure your neck using same tape measure to estimate how the length will look on you.