Since you seem to be inexperienced with larger dogs, I'd go with a great dane. They're huge and intimidating-looking, but have fairly mild and friendly temperaments. They're known to be relatively low-energy and can do just fine in apartments and smaller homes. Their bark is also comparable to the T-Rex on Jurassic Park. Seriously. It will shake your house. I don't think anyone would break into your house with something that size roaring and growling at them, whether they'll actually bite a prowler or not.