Length is going to depend upon your height if you primarily need it for walking. You want to be able to keep your arm straight and not bent. If the cane is a bit long, you can tilt it at an angle to make up the difference, but that won't work if it is too long.
For my current cane, I started with a hickory maul handle that was 36" long and then smoothed it down so that it was basically round (instead of basically oval as it came from the manufacturer) and gave it just enough of a gradual taper so that I could put a rubber tip on the end.
In my case, I just screwed the T-fitting onto the shaft of the cane. Since NPT threads are tapered, I had to taper that end of the cane too. The threads are a bit too fine and not deep enough in my opinion for use in wood, even though I have not had it come loose yet. A very course deep thread would be better, so if you happen to know a machinist that could recut them for you, that would be great. As it is, I have a feeling if you started beating on someone with the head of the cane, it would eventually loosen... Probably not until after you had caved his skull end though... Ultimately, I want to get a piece of 1" steel pipe about 3-4 inches long and weld it to the bottom of the T-fitting. The 1" steel pipe would have a bit smaller inside diameter than the shaft of my cane, so it would need to be hammered down on it which should keep it fairly well attached.