We offer our rings in 3 sizes: S, M, and L
Here is their correspondence:
S>51 to 52
M>53 to 54
L>55 to 56
The most reliable way to find out your size is to have your finger or ring measured by a jeweler.
You will find on the internet other tips to measure your finger either by its internal diameter or by its circumference, which remains a more approximate calculation.