I have heard two sizes for jo:
1. to your own armpit
2. to your own solar plexus
And for the Bo it should fit in your hand, your arm raised straight up in the air
(a bit longer than 180 cm
)
But where I got the measures from I do not remember
In my old Danish "dictionary" over Budo (from 1976), by a Karate guy, it says : Jo, a staff less than 150 cm. And Bo, a staff longer then 150 cm.