This is a simple ring in different sizes to test which ring size you need. The ring sizes are given as diameter and circumference.

16.9mm resp. 53mm 17,2mm resp. 54mm 17,5mm resp. 55mm 17,8mm resp. 56mm 18,1mm resp. 57mm 18.5mm resp. 58mm

Be aware, that if one of these rings fits well, you might still need to choose a smaller one, especially if the one you buy has rounded edges and/or is not as wide