Necklace size depends on neck perimeter, that why we have demonstrated minimum and maximum of each size. 
Necklace length in cm Necklace Length in Inch
38 - 41 cm 15 - 16.1 inch
42 - 45 cm 16.5 - 17.7 inch
46 - 50 cm 18 - 19.7 inch
51 - 55 cm 20 - 21.7 inch