For sailing (tiller pushed forward all the way) I prefer the tiller handle to be in may hand with just a slight bend at my elbow (i.e. my arm is almost straight laying along my torso). That sets length of the tiller itself.
Rounding marks may require pulling the tiller back a bit to allow for more turning, and for starting, most people like to have the tiller even longer so they can push from a more upright position. How far you can pull the tiller out is dependent on the length of the aluminum housing attached to the tiller post. There are various devices out there that allow the tiller to "lock" at the longer length and then release as you climb into the boat at the start (although some people find that just applying a fit of twist to the handle generates enough friction for the start).
If you're making a split-style tiller, I'd make the square end that slides into the aluminum tiller housing a few inches longer than the housing so you can trim the length to your particular sailing style.
Handle width is varies quite a bit. I don't have my tiller to check, but I like a handle with an opening wide enough so I can grab it with my hand (with sailing mitten on).
Don't forget to make some kind of stop so you can't pull the tiller out of the aluminum tube while you're sailing (I don't have one - I've never pulled my tiller out, but I worry about it...).
Cheers,
Geoff S.
P.S. I have some instructions on making a simple tiller at:
http://geoff-s.net/ice/FoamCoreTiller/index.html