Photos courtesy of Paazio
The story goes that Muso Gonnosuke, founder of jojutsu, received a vision from god during a prolonged fast at a temple. God apparently told Muso that he would be able to beat Musashi if he would use a stick of certain dimensions. The magical dimensions translate into metric as about 128cm long or into English as about 51 inches long. Serious, fanatical jojutsu guys are typically sticklers for this 128cm standard. There is no flexibility in the standard for smaller or larger participants.
I am not a koryu snob. I am more of the ideal of making jodo my own thing - taking ownership of the art - and I say that the standard is not magic. It seems to me that you need a jo to be...
- ...short enough that you can palm both ends and move your shoulders through a wide range
- ...6-8 inches longer than a normal 40-42 inch bokken
That seems to put the natural length of the jo between about 46" and 52". Within those confines, make the jo fit your body. If your shoulders are tight or you have a child practitioner, use a little shorter jo if you want to. I typically practice with a 51" jo, and the way it fits me is like this: