Any idea on taper or diameter?
I'd say something like a ball peen hammer handle would be nice 11-12" in length. The handle should slim down substantially near the top to allow the handle to have some give to it in use. Here's is one I made for an old coworker next to and old ball peen I used for inspiration.
You could also do a straight fawn's hoof swell at the end if the primary use would be the axe bit. It would be something different and still quite functional.