So once my ferrule is on I use the calipers to mark the depths at certain areas of the handle. Now this is a personal item but for starting out this is what I use for sizing:
The maximum diameter of 1.5" will be up at the shoulder near the ferrule, say about 3/4" back.
The narrow diameter of 1" will be up from the end about 1/3 of the total length. So on a 12" long handle the 1" mark would be 4" from the end.
The butt of the tool should be somewhere between 1 and 1.5" in this case I made it about 1 - 3/8"
So you can definitely toy around with these sizes to tune it to your hands or style. As you can see the final shape resembles a tiny little baseball bat.
So we have a nice shape but let's add some nice decorative touches to, at this point you can do anything, maybe add some finger grooves or some texture. I decided to make some burned grooves cause they look nice and improve the grip of the tool. I start out with a skew or a chisel point and make some light grooves on the shoulder of the tool. Once your grooves are cut take a coat hanger and gently push it into the groove while the lathe is spinning. Push the hanger till you get a good puff of smoke and check it. Make the burns as dark and deep as you like. I placed grooves at the front and back of the tool.