My 12.5 inches (32 cm) amber teething necklace fits most infants and toddlers between 3 months and 6 years. However, the easiest way to make sure is to take a measuring tape or even a piece of ribbon or string and loop it carefully around your child's neck to see what length fits the most comfortably.
It should not be so loose that the child can lift it up into his/her mouth (amber is not meant to be chewed on!), but it should not be so tight and uncomfortable. You should be able to insert two to three fingers between the necklace and your child’s neck. Also, note that amber necklaces will stretch in length over time because of the thread.