Belt fit comes down to one measurement: the working length of a belt you already wear.
How to measure
Lay a favorite belt flat and measure from where the strap meets the buckle to the hole you actually use. Match that to the length listed on the product page — that working length is how belts are sized, not the strap end to end.
Standalone buckles list the strap width they take — check it against the belt you plan to pair them with.