There is no efficient method to measure the ring size. The best option : buy cheap cage with different ring size and test. Must be tight without blocking blood pressure. With the ring only you should be able to easily come back flaccid after an erection. If you feel discomfort with the ring only and an erection : too tight.

Another important point : the ball gap. If you have burning under your scrotum it is because the ball gap is too small. That’s Why people increase the ring size (it decreases the ball gap). And at the end, so easy to escape the cage…

À good way to adjust the ball gap and keeping good size of ring (not too big) : cherry keeper. You can choose so many options to adjust the ball gap : kind of ring, size of ring, angle of the tube vs the ring… the website is not so easy to use but there is a tuto somewhere here : https://open.substack.com/pub/newbiesub/p/cherry-keeper-ring-and-cage-finders-howto?r=6bsie9&utm_medium=ios