Did O Sensei have a preference for the Jo?
I've read some Aikido Journal interviews with Ō-Sensei's direct students that claim he always referred to his jō as a bō. I've seen some theories that jō-length staves were used in training because there wasn't enough overhead space at the dōjō in Iwama for a bō. I personally don't find that to be a strong argument, as the late Saito-shihan records that most of their weapons training was outside.
My own conjecture is just that Ō-Sensei simply adopted the size of bō that his teacher, Takeda-sensei, preferred to use, who at 4'-11" / 150cm was even shorter than Ueshiba Ō-Sensei (5'-3" / 160cm). For added evidence: if you can find the Katō Shigemitsu footage of the Daitō-ryū bō subset in the gokajō set, you'll see that the staff they are using is suspiciously jō-length.
Additionally, there is film footage (2 or 3 different films, the Hawaii Aikikai opening is most notable) of Ō-Sensei using what amounts to a sharpened jō, and stories of him getting it out when he really wanted to make a point (pun intended). It is also known that Takeda-sensei trained Ueshiba in the Hōzōin-ryū method of yari (spear), but this was separate from the Daitō-ryū curriculum-proper.
Lastly, Ō-Sensei trained a lot of his students in jūken/mokujū (bayonet/wooden mock-rifle for bayonet training), and a jō happens to be about the same length of the Japanese infantry rifle of World War II. Bayonet training seems to have been mostly focused in the pre-war and war periods, but more defense-centric training (takeaways, mostly) seems to have persisted much longer (there might still be a few aikido dojo around that sometimes train with mokujū).