Bracelets: The easiest way to measure is
to take a bracelet that is the length you like, and measure
it. Keep in mind that very large beads will take up more
you, so you need to compensate for that.
If you don't have
a bracelet that is a good fit for you, the convention
is that you measure your wrist with a cloth measuring tape
(if possible) and add one-half inch to that number. For
example, if your wrist measures 7 inches, you would order
a 7-1/2 inch bracelet.
Necklaces: Again, you can measure
a necklace that you like the length of, or take a thin
rope (nothing too stretchy) and measure that way. Some
necklace designs work better at a short length, and some
are better longer.