this little trick will help you; cut a piece of string or yarn in the length you are considering, put a small pendant around it and hold it up around your neck as a sample and see if you like where it falls.
the easiest is to measure your ankle is with a flexible centimeter just above the ankle bone. you can also take a piece of non-stretchy string and then measure with a ruler. add 2 centimeters to this measurement. this will give you a pretty good size estimate for an anklet that falls below the ankle bone.
If you still aren't sure, feel free to ask me. I want to help you make sure you are happy with your choice! just send us an email firstname.lastname@example.org