Hi John:
I will draw on my years of experience demonstrating the art of traditional hewing at UCV to try and answer your questions. No doubt many will disagree with my comments but that is their perrogative to do so, and I welcome their comments.
First of all if you are using an 18 inch handle, in my opinion you are living alittle dangerous. As you work with a handle of that length the blade is a bit too close to the side of your legs, and if it glances sideways, which inevitably it will, it happens so fast your reflexes will not be able to stop it before damage is done. If you were a student of mine you would be using a handle of about 28 inches in length including the head of the axe. This will put the axe out in front of your feet, but you will still need to practice caution as you work, and take regular rest periods to maintain control.
The shape of the handle is another thing to take into account. I always used an oblong shaped handle, one that I could just close my hand comfortably around. It is imperative that your hand closes-- too big a handle will cause the muscles in your hand to tire out , and you will not be able to maintain a good grip which is important in the control of the stroke of the axe. Everyone has different shaped and size of hands so your handle will have to reflect the size of your hand as it grips.
I always give my handle a slightly upward curve, (never down), and when you hang the handle in the head of the axe, try and position it so that the cutting edge of the blade is parallel with centre of the log as you position yourself is a cutting stance. That way you can hold up a bit to give a slight bit of an angled cut or downward if need be in some instances.
It will take some time for you to feel comfortable with the handle and the axe itself. I suggest choosing an axe that is not too heavy or wide (10" max), and one that has a nice ring to the steel, and forged behind the inset tool steel edge and shows character and care on the part of the manufacturer.
I hope this helps you john and if you need additional help I have a home video of my hewing that might help you continue safely, --good luck, and have fun!!
NH