This Kidney belt is the traditional kind. It has a wide stretchy panel in back to provide the tension and then has wide fabric panels that come around front and attach to each other with strips of Velcro. We measured both sizes and here are the ranges we came up with.
Small-Large: 30” to 44” belly
XL-3XL: 42” to 56” belly”
These ranges would be around the navel area (belly).