Hi,
The lengths of various staff weapons are not necessarily fixed in stone, for example the Jo, at certain times in history, would be personalised to be the height of the users armpit, which could easily be 5 foot if the wielder is 6 feet or taller. As well as that, the Portuguese art of Jogo do Pau uses a staff of between 4 and 6 feet, obviously including the five foot length, and the same applied to European Quarterstaff.
With regards to our weaponry choices, my instructor actually wanted to use a 5 foot staff for Jo, however I disagreed. His reasons were that, being Westerners, 5 foot was a better "fit". Mine included the height of the ceiling in the room we were having, as well as the relative lengths rather than the specific lengths of each weapon.
In short, each length has it's own characteristics and properties that separates it from the others. A Tanbo (short staff, two feet) it typically a single hand weapon, used for striking with some grappling as well. Hanbo (half staff, three feet) begins to involve two handed strikes and grappling methods, and is a close-quarters, in-fighting weapon. Jo (stick, four feet, although some schools do use a five foot version) is just beyond the in-fighting range of Hanbo, being focused more on two handed strikes for power. Bo (staff, six foot) is a ranged, two-handed weapon.
It's important to notice the proportions that increase, rather than get fixated on the lengths themselves. If we take the Bo as the basis, it's a six foot long weapon. The Jo, at four foot, is two-thirds of that length, being a big enough change to justifiably alter technical approaches. The Hanbo is then three-quarters of a Jo, or half of a Bo, again a big enough difference to make a difference. If a five foot Jo is used, it is simply too close to a Bo when compared to a Hanbo to realitically give a different "feel" and approach.
To take it the other way, a Hanbo is three feet. To go to a Jo, you add a third of it's length. And to go to a Bo from there, you add half the length again. If you use a five foot Jo, you add two feet to the Hanbo, nearly it's entire length. And to then go to a Bo, you only add a foot to the length, which is just 1/5th of the entire length, not much of a change. If you were wanting to use a five foot Jo, you should also be using a Hanbo of just under 4 foot, and a Bo of 7 and a half feet. So provided that's what you're using, 5 foot will make sense. Otherwise, if you train with all of the above weapons, the typical lengths are best.
