The ultimate way to measure is if you or your husband/wife has a vernier calliper in the toolbox.
Then you easily measure the innersize of a ring you know fits perfectly.
Otherwise you can put the ring over a folding rule and with your eyes see your approximately size (not very reliable).
You can drop by the nearest goldsmith store and ask them to measure your exact size.
Certain bracelets need to fit pretty tight to be worn neat, as for example very thin chains or broad leather bracelets, while other designs can be worn more loose.
Thick bracelet chains needs to be a little longer.
Measure your wrist and add about 1,2-1,5cm if you want a tight fit, or add about 2,5-3cm if you want it to be more loose.
You can also easily measure a bracelet you already has, but don´t forget to include the lock.
The easiest way to find out a perfect size is just to measure a neckless you already own, (and include the lock). Otherwise you can take a tape measure or a string and thereby measure the length. Keep in mind that certain neckless designs can need a specific length to be worn neatly.
Please contact me if you have any questions.