Take all the length it will give you (70" should be plenty), and all the width it will give you (your width will be determined by the diameter of the sabling, rather than what you want.) Thin the whole thing to about 1" thick, except the handle area. Keep that thicker.
Is it dry? If not, restrain nthe wood to keep it from laterally warping or twisting. White wood saplings will give you fits as they dry.
Make a long, narrow flatbow, (as wide as the stave will allow) starting with no more than 2" reflex (because it's so narrow), and start narrowing side to side about 1/3 of the way from the tips.
Good luck.