Pls measure and cut down the required size from a string or a thread for the finger/hand that you wear your ring/bracelet. And measure the string/thread using a normal 30cm ruler. Just as the pics below;
And in your order, pls indicate the length required. Standard size in this store is 6.5cm for ring and 16cm for bracelet.
Example;
Size for bracelet: 17.5cm
Size for ring: 7.5cm