Each rooster is different with his own personality. Some are better flock protectors than others, not because of size but because of personality. To me, a good flock protector watches for problems, warns his flock to run for safety, and puts himself between his flock and the threat. I had one that would lead his flock to safety. To me, that was not acceptable. He should send them to safety while he protects the rear.
They do not have much of a chance with a lot of predators as far as having a chance to win a fight, though occasionally they will. Even if they are alert and watching, a fox or something like that can attack from ambush and be gone with the hen before the rooster can react.
A good rooster will do what he can to protect his flock, even put himself at risk, but he certainly has his limits.