Probably not a small dog, as they might accidentally get crushed if Birdie wants to play. I know that my dog (Manny the English Bulldog) always thinks of small dogs as his own personal chew toys, so I have to keep him from them. On the other hand, you probably don't want a dog breed that is similar to a Mastiff, because there might be a territorial thing there. How about a Golden Retriever? They grow big enough to where they'd be able to hold their own in a wrestling match with a Mastiff, but they have the nicest temperament. Either that or a Lab, that's what I'd go with. Oh, but those kinds of dogs are really active, so if you'd rather have more of a couch potato dog, a Bassett Hound might be a good choice too.