How to tell the image circle size of a lens?

I have a 210mm CP Georz Berlin Dagor lens. How can I tell the size of its image circle? Is there a test I can do, or does someone know if there is a standard image circle size for these lenses?

Thanks.

Turn out the light in the room. Open the lens to full aperture. Place a large piece of paper (white) behind the open lens. Point lens out the window. Move paper back and forth behind the lens till you get as sharp as possible an image on something at infinity.

Measure the size of the circle the lens throws onto the paper when the lens is at infinity.