As a very, very rough rule of thumb, I recommend that the bo be somewhere in the same neighborhood of your own height, give or take 3 inches.
This is for your "average" person. I realize that there are many people who may have longer or shorter arms, or perhaps they simply prefer a certain length. Other times, perhaps people who want more dense woods, such as Jatoba, Purpleheart, Ipe, etc., may want a slightly shorter bo, since it's going to weigh more. Then again, maybe they want it longer AND heavier.
I can't argue with any of those reasons, and in the end, it's really up to the user's preference.
I can, however, get them somewhat comfortable, so that the length isn't the main issue. If I see someone struggling with a bo that's too long, I'll recommend they get it cut, or that if theirs is too short, then they should relegate that one to becoming a jo, and get a new bo that fits them.