I look at it this way.
It’s not that the dog needs to be physically dangerous. If the dog is big, a potential intruder is going to be intimidated because he doesn’t know how dangerous the dog may be.
A guard dog should be large enough for size to provide some intimidation. It needs to be well behaved except when it sees its territory encroached upon. Dogs that bark indiscriminately (or all the time) are worthless.
The dog should let you know when the bad guy is near & you should have the proper firearm to do the job, not rely on the dog to disable an intruder.
Agreed.