Just like many working breeds, there are breeding lines that are better suited for work than others. Not every Doberman can or will work, but if you find the right lines, they can be hard working dogs. We had a woman come to our dog club recently with a 1 year old male Dobe. There was no way he would work. He was fearful and terrified of men. Obviously he had some issues, and obviously there was no way he had the temperament or genetics to work. He was a nice looking dog and all but the mental part just wasn't there. Another girl who I train with has a nice Dobe male who is good sized, very athletic and graceful in his movements, and probably could at least pass the evaluation test for our club.
As far as build goes, our Dane looks like a fragile dog, with her long legs and long neck and thin build. I tell you what, she is one of the strongest dogs I've ever seen. Don't let looks deceive you.
I don't see much difference between medium sized dogs like Mals and some of the GSDs, and the larger dogs like the Bouvs and the bigger GSDs at our club. The smaller dog isn't always quicker and more agile, and can't hit as hard. The male Bouv and Gunnar are both dogs who are larger, but are also fast and agile, and hit like a tank. When you hear a nice thump when one of them hits the decoy, and they drive him to the ground, you know they are hitting hard. I have yet to see the smaller GSD (who is from Czech lines and is a VERY hard dog, but is only 67lbs) or any of the mals do that.
RD, regarding your question about how a protection dog will hit vs a sport dog- a protection dog will hit the center mass of the target. In training the decoy will try and sidestep the dog, so they learn not to fly through the air (what makes a Sch dog look so dramatic), but to slow down a bit, hunker down, and then target the center of the target. That could be the middle of the back, the butt, legs, armpits, whatever is there. That's what they are trained to do. Even if an arm is hanging out, they hit center mass. There is definitely a technique to catching a dog on the sleeve when he's learning to target center mass. You need to absorb the dog into you, as you can injure the dog if you resist him. It only takes one bad catch to jam a dogs neck and put him out of protection training for good.
There are dogs that succeed in Sch who could do real protection work, but not all of them. Sch never really tests a dogs limits. Sch is a choreographed routine. Same thing every time, and the decoys are there to make the dog look good and play them up, not break them down and drive them off the field like a French Ring or PSA decoy tries to do. I would say that ANY dog who can do true protection work can do Sch with an adjustment to training methods- the stability and temperament is there in that kind of dog.