first off, if your hickory has good grain then its a better backer, you could back oak if you want, or you could back maple. If your intent of making hickory the belly then i suppose maple would be a good one, HARD maple though. Dimensions? um 1 1/2 inches wide from fade to about mid limb and then taper to 1/2 at the tips. you can go wider if you want but that's just a suggestion. The key is to make the backer just thick enough but not too thick. In this case, maple might have to be a tad thicker than if its a hickory backing because hickory is a very strong tension wood. something to remember while gluing up, make sure that you do a dry run to see where everything will wind up. clamp it together and see if there are spots that you can see through, if you can that could be a problem. Ash is another wood that seems to be really quite nice in tension. if you have the desire you could back a wood with ash. If you pick a good combo the wood can be less wide over all. hickory backed something(not sure whats great in compression) would be an excellent bow, it would probably be walnut or something like that for a strong belly, but if you don't want to buy that wood or you just want to keep it simple, then go ahead and buy some kind of hard maple, ash, oak, or something to go with the hickory.