I agree that the type of dog is not that important when it comes to scaring off would-be intruders. The important thing is that they alert you if something is off. A determined burglar who wants to get in your home can take care of any dog fairly easily. But, most theives are looking for an easy target, and homes with any dogs are more difficult than a house without a dog. If someone is checking your house out and hears a dog barking from inside, that alone is a deterrent and they would hopefully want to pick an easier house.
Strange dogs can be intimidating just because you don’t know what they are going to do. We have a shepherd mix, about 40 pounds. She would be no good as an ‘attack’ dog, because her response to strangers entering is to give them kisses and madly wag her tail. She does bark at people outside though, and I do encourage her to do this. Not incessantly - she will bark to ‘tell’ us something is out there, when I go and see the person (or squirrel, or trash can that was not there before) she is praised, then told, ok, enough. She is usually satisfied that we, the pack, are aware of the presence and will stop barking then. (Except squirrels - they are a particularly threatening enemy and must be scared off. If a squirrel ever tries to break into our house we are all set.)
My husband has a truck for sale right now and told someone they could stop by and look at it. He came by when we weren’t home, and told us later he ‘saw your dog and didn’t want to look around too much’ on his own. The dog was inside, the truck was in the garage - not a threat to him but still a deterrent. This is exactly the response we want (even though the thought of our dog intimidating anyone is hilarious to me) - she makes her presence known and watches what is going on when we are not there. You can train about any type of dog to do this, so I would concentrate more on finding a dog whose temperment fits your lifestyle, you can train it to guard later.
