Any size will work, or even a rock. The reason the starter stops working is the brushes wear to the point they no longer make contact. By banging on it, the brushes will move enough to contact the commutator again. But it only works a few times. Luckily it worked again when unloading from the ferry!?
LOL been there done that!, except the ferry truck had to push me off so I could fix it in the parking lot.