You do not need to measure your neck for necklaces. They are designed to fit most of us, though you can always consider the length for the purpose of layering.
Women's Standard Length Necklace Sizes:
Collar 30 cm / 12 inches
Lies flat to the neck rather than hanging freely.
Choker 40 cm / 16 inches
Sitting closely around the neck.
Princess 43-48 cm / 17-19 inches
It will fall just below the throat and sit at the collarbone.
Matinee 50-55 cm / 20-22 inches
Will fall below the collarbone or sit at or above a low neckline.