At some point, with all the possibilities of thread form, diameter, and pitch, you need to send it off to SK Grimes or another machinist. You could buy the thread pitch gauges and comparator and thread micrometer (to measure minor diameter) to fully define the thread, but that still won’t help you to find a ring.
Place one end of a tape measure on the inside of the flange. The diameter is used to determine what size pipe a flange can be used with, so you must find this information by using the inside of the flange as opposed to the outside.
Stretch your tape measure across the center of the circular shape of the flange until it touches the other side of the unit. Keep this end of the tape measure on the inside of the flange as well. Because the flange is circular in shape, the tape measure must stretch across the exact center of the flange to provide an accurate measurement.
Read your tape measure, in inches. The distance you have found is the diameter of your flange.