The best way to determine the length for you is to take a crossbody you already love the length of on you. Then measure it from the top of the center of the bag to the apex of the strap when taught. Here is a general height guide, but of course, you are welcome to pick the length you prefer since everyone has different preferences!
Click here for more for images of both Full Size and Mini size bags on a 18", 20", and 23" drop length chain strap on models at various heights.
Size Guide for Strap Lengths According to Height: