Below are the sizes for the diameters of the various NW, QF, KF and ISO vacuum fittings.  We have included a file you may download that shows, to scale, the various sizes.  This chart also lists the physical measured diameters of the flange so you can use it to review what you have there using a ruler or calipers.  There is typically enough differentiation in the size of the diameters of the flange compared to other size of flanges to easily determine one NW size from another by simply by measuring with a ruler, scale or calipers.  Exceptions are some of the odd balls.  The NW10 size, which is not often used, is many times satisfied with standard NW16 parts.  If you download the NW Size Chart, it is a PDF file and should be at a 1:1 scale, so you simply put your vacuum fitting on top of the image on the chart and there ya go.

NW Size Chart