If dogs could fly, how big and what kind of wings would they have?

So if a large breed dog could fly (German Shepard, Golden Retriever, Labrador, etc.), how big would the wingspan need to be to keep them in the air for sustained periods of time?

Would their wings likely be bat-like, feathered, or even crazy-fast and nimble like a humming-bird or insect? Is there a formula that biologists use to figure out wingspans to bodyweight? Would the flying dog be able to hover, or just glide? What would you name your flying dog?