I have checked this again and have to tell you that there is no sizing chart from the manufacturer. The easiest way to find the perfect size is to buy a sizing kit: RingConn Smart Ring Gen 2 Sizing Kit
Alternatively, you can also contact the manufacturer directly, they may have more detailed information on the sizes: https://ringconn.com/de...
If you order directly from the manufacturer, the sizing kit will be sent to you in advance free of charge.

Thank you for your answer.