Looking for the perfect chastity cage

I’ve been battling to locate the perfect chastity cage. My cock is thick and average in length when erect, but once I put a device on it goes flaccid and pulls back completely so that only the ring remains attached. The unit rotates freely.

How do I determine the correct size?

You can find plenty of advice on supplier websites; for instance: There is also considerable guidance available here if you take a look around. Many of us have found that going smaller works best.