Some of the necklaces and bracelets have various size options. For necklaces, the most common length for women is 16.5". The most common length for men is 21.5". For bracelets, the most common size for women is the 7.5" and for men is the 8".
If you are still unsure about what size to buy, use a piece of string to measure the size of your wrist. Make the string fit like you'd want your piece to fit. Measure your string with a ruler and choose the size that is the closest to that measurement!