I'm no expert, but I don't think there's such a thing as too big when it comes to personal protection. It's all about defensive work. I think that a giant mastiff is just as good as a smaller dog, if not better. Larger dogs are certainly more intimidating-looking! And it would certainly be harder for a criminal to get through your dog to get at you.
But if it comes to actual apprehension work, like chasing down bad guys, I think size is definately important. While a 280 lb mastiff is intimidating as a personal protector, they just don't have the agility to clamber over high fences and slip through narrow fence cracks in pursuit of a felon.
I remember seeing a video posted on here showing Belgians (probably weighing 60-70 lbs) doing some absolutely amazing apprehension work. Completely vaulting cars to get at the suspects hiding behind them, leaping into moving van windows to latch on to a gunman's arm, and more. It was all done at astounding speed---they looked like bullets. I don't think a fila or bullmastiff is capable of performing like that.