Breed or size is secondary to the issue at hand. What counts is, will the dog do what you need him to do when it is time for him to step up and do it?
I once read an excellent analogy of what we are looking for in a young dog that we wish to use for such work:
Let's say we have a young athlete in superb physical condition with good intelligence...a swimmer or tennis player perhaps...good reflexes, sharp eyes, all the right tools...but he has never been in a fight in his life.
Let's set him down in a boxing ring with an experienced Golden Gloves contender. When the bell rings, he gets knocked flat on his ass by a right cross from our boxer! At this point, one of two things is going to happen; he is either going to say "I quit, you hurt me!", or he is going to get up and do his absolute best to give the other guy a dose of his own medicine.
You see,we can teach him HOW to fight...what we cannot teach is that willingness...even desire...for the fight. That must come from within.
Yes, there are other considerations where dogs are concerned, and training of the correct type from experienced people is certainly going to be required. Still one of the great "secrets" of dog training is to first pick the right dog.
There is much information out there on these topics, starting here on Ed's web site. Do not depend too much on your dog or any other single element of your security plan, rather, layer a "defense in depth", so if one layer is breached, they will be facing another and another.
This world we live in can be a very scary place, but we need not be terrified to go about our daily lives so long as we stay alert and aware of what is happening around us and use a dose of common sense. (Life can be both short and violent for people who do really stupid things!)
Last, a little plug for our cops:
Every day and night, all over this country, good men and women take incredible risks with their personal safety so the rest of us can sleep without posting armed guards on the perimeter. They not only risk their safety, but the very lives and futures of their families and loved ones.
They fight a thousand, thousand little battles each day, and we only hear about those few where someone's actions were less than perfect. Yet, who among us has made not one single mistake?
They, too fight a war. The battleground is, literally, our own back yard. It is squalid apartment complexes, stinking alleyways, high-dollar subdivisions, lonesome stretches of highway, farms, fields, woods, the street in front of your home. They see things no one should have to see, and do things no one else will.
If this war is ever lost, the very future of this nation will be lost with it.
Do not make overmuch of them...they are citizens like all of us. They know joy, sadness, courage and fear just as you do, and most are there because it is where they wish to be.
But save a small place for them in your hearts, and think about them now and then. For they, too, suffer casulties. They, too are warriors. They, too deserve your respect.