To do this measure your wrist and add 3 x the depth of the chain maille.

Now you need to add some length to get the fit you want. This may be just enough room so you can move your hand freely or it may be longer if you want your bracelet to dangle down onto your hand. This measurement must include enough length so the clasp can be easily done up, this will change depending on the type of clasp and how soft or rigid the materials of the bracelet are.