Stave generally refers to a piece of a log split down to a size where it will build one bow. A board typically refers to a piece of wood which has been milled down to a rectangular cross section, like you'd find in a lumber yard.
Generally you will want 2" wide or so minimum for a white wood flatbow stave (ie hickory, maple, oak etc) by whatever length the bow will be plus some for leeway. The depth should probably be minimum 1.25" so you can make the handle stiff. Worst case scenario you can glue another piece of wood onto he belly side of the handle to deepen it.
As Macbow said, you can use the outside surface right under the bark as the back of the bow for many woods (hickory, maple, ash, oak, elm, yew and many others). Some woods require what's called "chasing a ring" examples would be Osage, mulberry, and black locust.
Regarding hickory vs maple, Either is fine, but if you choose maple it should be "hard" maple, also known as sugar maple, or rock maple. The other maples are much less strong. Hickory is a great go to wood. Very tough stuff. Make sure it is dry before you bend it at all, and probably keep it somewhere with relatively low moisture because hickory will soak up a lot of moisture from the air, the bow will weaken and take more set.
Read the TBB vol. 1 and a lot of this will make more sense.
Cheers