Ergorig

The regular size will fit the majority of operators, but if you have a shorter torso, go for the small. Measure from your front hip point to the top middle of your shoulder. If you're less than 21" order a small. Your front hip point is the part that feels like the tip of the triangle in your hip on the front side that is right around your belt line. Of course there's overlap between the two sizes, so if you're right at 21" either will work.

The only difference between the two sizes is the length of the struts- the belt size is the same.