Sizing your wrist is simple; use a tape measure and measure around the part of your wrist just above the joint. The bracelet will be that measurement plus the chosen clasp, choose the bracelet size closest to your wrist measurement. teh toggle clasp will result in a looser fit.
No measuring tape? use a piece of string, wrap the string around the wrist the same way, hold your finger where it ends and then measure the length of the string on a ruler.
If the bracelet is a gift and you do not know the size, the average bracelet size is 7", a small would be 6-1/2", a lrage 7-1/2" and a nextra large would be 8".
This is approximate sizing methods and not a guarantee of an absolute perfect fit.