Titanium steel ring: Small, Medium and Large are the diameter of the top face of the ring in millimeters — Small is 16 mm (0.63 in), Medium is 24 mm (0.95 in), and Large is 30 mm (1.18 in).
Solid 925 silver and solid gold ring: Small, Medium, Large, X-Large and XX-Large are the diameter of the top face of the ring in millimeters — Small is 12 mm (0.48 in), Medium is 15 mm (0.60 in), Large is 18 mm (0.70 in), X-Large is 21 mm (0.82 in), and XX-Large is 24 mm (0.95 in).
They are not your finger's ring size; you choose your US ring size (US 4 to US 18 in half sizes, or a free ring sizer) in a separate step of the customizer.