In this page you can find away to measure the size of your jewelry.

Only for the ring sizing, print this ring size guide on a sheet of A4 blank paper on landscape mode.
Then check the inch guide on the right side with a ruler to make sure your printer has preserved the scale of this guide
Take a ring that fits your and place it over a circle. Find the circle that just touches the inside of the ring.

Measure your wrist at the wrist bone.
Add 1/2" until 1" from your wrist bone size.
(This measurement only for bracelet with chain style)

For a quick reference, here are the typical bracelet size :

When selecting necklace size, it's important to consider how do you want to wear your necklace jewelry. It's all depend on your style.