For necklaces you can use sizes from 35 cm (choker) to 85 cm (long necklace).
Bracelets for smaller wrists measure approx. 16 / 17 cm. An average wrist measures approx. 18 / 20 cm. A larger wrist will measure approx. 21 / 23 cm.
The correct size can also easily be measured with a tape measure.