Dogs are awesome friends and can be excellent protectors. Personally I would not want my firend to be put in harm's way to protect me or mine. I rescued two Chows from the local doggie prison and they have proven to be highly effective noise makers.
They hear what I can't hear and see things before I can. All I have to do is pay attention to them. Not certain size matters unless it is a Working Dog. I would look for dogs with a naturally protective nature. If they are big, then it is all the better.
Otherwise, get a good noisemaker, let them roam the house at night and keep your firearm on your person or within arm's reach.
Best of luck!