Rhodesian Ridgebacks are large, protective, yet loving dogs. Might be an option. My family had two German Shepherds when I was growing up (one that was rejected from a seeing eye dog school b/c it got too big to fit under a bus seat). Great dogs but it always breaks my heart to see them struggling with their nearly inevitable hip problems. You may also splurge on a Labradoodle--big enough to be intimidating, friendly like a lab, and smart like a poodle, plus no shedding! Here's our labradoodle, Argus, who barks at the slightest sound (though he's 3/4 lab and sheds).