Smith&Dillon Size Guide 

Size 6 = Ring Sizes L, M, N. Inner Diameter measures 16.5mm

Size 7 = Ring Sizes N, O, P. Inner Diameter measures 17.3mm

Size 8 = Ring Sizes P, Q, R. Inner Diameter measures 18.1mm

Size 9 = Ring Sizes R, S, T. Inner Diameter measures 18.9mm

Please also note that if you are ordering a wide band ring, its usually a good idea to go up a size. If the ring falls between two sizes, order the larger size.