The measurement given in the item ID, or the part number, is the inside measurement of the hardware. For example: if the mearurement of a D-ring is 3/4", then that is the the measurement of the leather strap that will fit inside that piece of hardware.
If you are looking for 4430 2 1/4" Brass Double Tongue Buckle, then 2 1/4" is the size of the strap on the belt you should be using.
The scale found below each item, or as a secondary image, is meant to provide you with a reference as to the overall size of the piece.