I make this dummy Oura rings for testing which size is the best for your finger. I make it with the table at Reddit:
Size: Finger diameter / Diameter with sensor
6: 16.76mm / 15.21mm
7: 17.60mm / 15.97mm
8: 18.43mm / 16.81mm
9: 19.25mm / 17.63mm
10: 20.02mm / 18.41mm
11: 20.81mm / 19.24mm
12: 21.64mm / 20.05mm
Tested at size 8. Printed with PLA. My step files are created with FreeCAD