From a ki-info post, Feb. 2000:


I've made a few jo's so I'll give you my thoughts on it. Traditionally
they are usually 7/8" to 1" in diameter (22mm to 25mm), and sometimes as
much as 1 1/8" (28mm) in diameter. Most of the ones that I have made
have been 1 1/8" tapering to about 1" at the ends, and octagonal. This
is not the traditional way of doing it, but it makes a stout jo with a
nice feel to it.  The traditional way is to make it round and not
tapered. The length is usually 48" to 50" (1230mm to 1280mm) or longer ,
depending on the persons height. Since you're making it for yourself, if
you make it so that it just fits under your armpit when you are standing
without shoes on it should be just right. 

The usual Japanese ones are made from Japanese white oak, which is
evergreen, so it has a very dense grain structure. I think it is similar
to our live oak which is also evergreen. I'm sure there are probably
some native woods that would be suitable for you to use, but hardwoods
are not all equal in strength. I believe there was an article in "Aikido
Today" recently that had a good discussion of wood selection for martial
arts weapons. Choose air dried lumber if possible, rather than kiln
dried, because kiln drying will weaken it (more or less, depending on
how skillfully its done).

I don't like to put any finish on the wood. Some finishes feel sticky in
hot, humid weather, others get too slippery when your hands get sweaty.
The natural wood surface seems just right to me.

Brian Kelley