The ideal length for the jo staff. The jo which is shorter than the bo and from what I know, generally speaking, 6 feet is the cutoff point. If your staff is 6 feet or longer its a bo, if its shorter than that its a bo. Anyway, the ideal length for the jo depending on the person's size, I read that when you place the bo up and down with one end on the ground the other end should come up to your armpit. So depending on how tall you are that's how long your jo should be although Im not sure how steadfast this rule is. In some ways, it seems a bit long.
Yeah... as Paul suggested, you don't even know how much you don't know yet.... and you still insist on phrasing things as if you're presenting actual knowledge. This is exactly the type of solidifying a lack of knowledge based on minimalist information that I was cautioning against in the other thread.
But, to add to Paul's comments, "bo" 棒 means a "stick or rod", basically a rounded straight length, which is often wooden but not always (one little thing, Paul... the "boku" in bokuto/bokken isn't the same... it's 木, pronounced "boku", "moku", or "ki", hence "bokuto", "kidachi", and so on... of course, the left radical of "bo" is the kanji for "wood - boku", so there is obviously the connection). "Jo", on the other hand (also pronounced "tsue/zue") is 杖, and basically means "stick". As a result, "bo" is often used for longer "rods", but even that isn't definitive...
The founder of Shindo Muso Ryu jojutsu, Muso Gonnosuke, was a practitioner of Katori Shinto Ryu and Kashima Shinto Ryu, and specialised in the use of rokushakubo from these traditions. When he developed the new weapon (due to a divine guidance telling him "Maruke wo motte, suigetsu wo shire"/"With the end of a rounded stick, understand and know the Suigetsu/water on the moon"), the intention was to be shorter, and lighter than his long staff, in order to be faster and more versatile... and the sizing was set as Paul listed above. The same is used in ZNKR Jodo (Seitei Jodo), as it was developed directly from Shindo Muso Ryu by Shimizu Takaji sensei.
Other arts, though, such as Muhi Muteki Ryu also use the term "Jojutsu", but their weapon measures 5 shaku 5 sun... essentially 5 and a half feet... almost what most would class as a "bo", although the school themselves refer to it as a "jo". Yagyu Shinkage Ryu, in some lines, teach Jubei no Jo, which is about 3 to 3 and a half feet long, and is made of a solid iron core with bamboo around it... one hell of a thing to get hit with... then we have various Tanjo (short jo/short sticks) found around the place... the most common being a two foot length of wood, although the Uchida Ryu Tanjojutsu, an assimilated school taught with Shindo Muso Ryu, use a tapered three foot long "walking stick".... the art was developed to take advantage of the popularity of the new "Western" walking sticks, and to help with the popularity of SMR at the time (the curriculum was originally referred to as "su-tekki-jutsu", which is basically the Japanese sounding out of "stick-jutsu").
Then you have specialist uses, such as Shikomi-zue (prepared stick), and length of staff weapon with a secondary weapon hidden inside, which may be a chain, a sword blade, a spear tip, or anything else. You also have Hanbo (half-staff), typically referring to a three foot staff (half a six foot one, really), Tanbo (really the same as a Tanjo, just with different preferences based on the school in question), Te-giri-bo, a short stick found in Asayama Ichiden Ryu of about 24 cm, and having a diameter of up to 15mm. Yagyu Shingan Ryu refer to their long staff as Cho-bo (basically "long bo")... but is only around 5 shaku long (note: the Edo line use a 6 shaku weapon, it's the Chikuosha/Sendai line who have a shorter weapon).
Add to all of this that, in many classical ryu-ha, particularly when looking at sogo bujutsu (composite schools teaching a wide variety of skills and weapons), the bo is not often developed as a weapon itself... it's commonly what is left when a longer bladed weapon (pole arm) is broken in combat, such as a spear or naginata. Within the Kukishin traditions, a rokushaku bo is what was left when the blade of a naginata was broken off... same with Toda-ha Buko Ryu, and Tenshinsho Den Katori Shinto Ryu. Other schools have it as a broken spear, and a jo as a broken naginata, and so on... as a result, the exact placement of the break can determine the length of the remaining staff... which became formalised for a particular tradition over time.
So, what's the length of a Jo? That depends... what school are you studying?