Most motocross kidney belts follow a similar sizing structure as motocross pants falling into an inch measurement system, kidney belts contain a lot of elastic though and most adults will fit into one or two sizing options.
When purchasing a kidney belt ensure you can secure the belt firmly but make sure it is not so tight that it restricts your breathing. A kidney belt should sit around your lower back in between the bottom of your rib cage and the top of your hips.
Kidney belts are either a single item or can be part of either a Back Protector or complete Body Armour pressure suit which provides protection for most of your upper torso!