The rule of thumb when measuring proper size is to not have the necklace so tight like a choker, but close to the neck where one finger can be inserted in between the neck and necklace. These necklaces are to never be long enough where a child can pull it over their chin and put the beads in their mouth.
Babies under 2 years: 11 inch-12 inch
Children 2-6 years: 12.5-13 inch is recommended. Again, know your child (big or small for their age) and a quick measurement never hurt
Adults do vary so make sure you measure yourself for a proper size.
*note- baby/toddler sizes are made with "break-away" clasps. If your child gets caught on something the clasp is made to "break-away" without any of the beads falling off to prevent choking or strangulation. If the clasp happens to "break-away" it is very easy to fix with pliers. Please note if you do not want this type of clasp. It is recommended for children up to 3 years.