To determine the size of a flange, you will need to measure the diameter of the flange.  Here's how you can do it:

1.  Measure the outside diameter (OD) of the flange: Use a measuring tape or a caliper to measure the distance across the outer edge of the flange.  Make sure to measure from one side to the opposite side, passing through the center.

2.  Determine the flange size: Once you have the measurement of the outside diameter, you can refer to a flange size chart or consult the manufacturer's specifications to determine the corresponding flange size.

It's important to note that flange sizes are typically specified in standard dimensions, such as inches or millimeters.  Additionally, some flanges may have specific designations based on their intended application or industry standards (e.g., ANSI, ASME, DIN, etc.).

If you're unsure or need accurate measurements, it's recommended to consult a professional or refer to industry standards for precise flange sizing.