Updated: the dog I babysat wasn't this big, but he certainly made our German Shepherd look tiny. He really is a "gentle giant" as the breed is called - very laid back. He and his "brother" (a much smaller dog) went back to their owners before I thought to get a picture. The owners are friends from Virginia just now moving out here.
ONE MORE THING: my sister's husband's sister severed part of one of her fingers when her mastiff puppy pulled on a wiry leash that got wrapped around her finger before she could stop it. They may be cute, they may be gentle, but they are BIG. Just something to consider if you were thinking about getting one. Keeping a hundred plus pound "puppy" under control might be difficult.