BPs usually soak for only 3 reasons: they have mites, they don't have enough humidity, or they are using the water bowl as a "hide" because they don't have anything else they feel secure in.

You can use a very small water dish, as long as it is big enough the snake never runs out of water. However, if you switch to a smaller one, it will likely make your humidity drop. You made need one with a large surface area to keep the humidity up.