The measurement given in the item ID, or the part number, is the measurement of the material that should be used with the particular piece of hardware.  For example; #79 3/4" swivel snap should be used with a 3/4" strap.  A 4430 2 1/4" Brass Double Tongue Buckle is designed to be used on a 2 1/4" strap on belt.  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.