Go with what Bubby said. You can even go wider, but 68" is nice and long.
I'm assuming you have a 4/4 board, about 3/4" thick. You'll want to add a handle block (or "riser") to build up the grip. Adding another 3/4" board will give you 1-1/2", which is about perfect.
First, mark the physical middle of the bow, place your middle finger on that mark, and make the handle five fingers wide; the middle finger, plus two above and two below. That'll be 4-5" for most people.
Narrow your handle to 1-1/8" wide or smaller, and maybe leave a little lateral bulge like Bubby said mid-grip. Looking from the front, make your limbs flare from the narrow handle to the wide limbs 45 degrees or less.
Make your fades (thickness taper from thickest handle to flat limbs) 2" long or so. Flares will overlap the fades a bit, but fades should extend out longer by way more than an inch.
Side taper the limbs from full width to 1/2" or less at the tips. Leave yourself some wiggle room.
Once it's all marked out, work the limbs down to about 1/2" thickness from the fades out to the tips, and start tillering from there.
