You can refer to the following table to find out the right size for yourself. This table is also provided on Najo’s website. The rings can be resized afterwards.
Size USA British Inside Circumference Inside Diameter
X small 6 L% 52mm 16.5mm
Small 7 N% 55mm 17.5mm
Medium 8 P% 57mm 18mm
Large 9 R% 60mm 19mm