In Kendo, the Shiani sizes are determined by the age and the gender of the person practicing Kendo. This is determined by the IKF ( International Kendo Federation) rules and requlations. The weight and the length of the Shinai is measured at official competitions. Please refer below to the IKF Official requlations:

Ages	Size
4-6 years old	Size 28/30/32
7-9 years old	Size 34
10-12 years old	Size 36
13-15 years old	Size 37
Adult Women and over 13~15 Year old	Size 38
Adult Men	Size 39

Click Here for IKF Shinai Size chart