There is a rule, and exceptions to it when deciding the size of a jo (medium length staff used in Aikido), Jodo and other sword/staff practices. The rule (and this comes from ancient times) is that the jo is cylindrical in shape and has specific proportions of 50 1/4" x 15/16" . Traditional schools of Jodo and the jo used in the Shindo muso ryu go by this rule.
So, old school dojo (koryu), Jodo and Shindo Muso Ryu practitioners should select the 50 1/4" x 15/16" size. For most aikidoka (Aikido practitioners), a more customized approach is taken which is technically an exception to the rule but since the vast majority of jo usage occurs in Aikido, the customized approach is more largely represented and the jo is sized according to body proportions as described below.