These are highy quality O-Rings.  The size is measure as the inside diameter.  So the 3/4" ring has an inside diameter of 3/4".  All rings except for the 1-1/2" have a 4-1/2mm ring wire diameter. This is the metal itself being 4-1/2mm wide.  The 1-1/2" ring has a wire diameter of 5mm wide.