How would your dog get attacked by a dog when you are not around? Your dog should not be outside the house or in the yard when no one is home. If there is a problem with strays, then your dog should ONLY be in the yard when you are right there, and only long enough to relieve himself, then right back inside the house.
It won't matter the size of your dog, if two or more dogs get in a fight, there will be a loser. Unless you want to be part of dog fights, keep your dog inside and don't worry about the actual size of your own dog.
If you walk your dog, keep pepper spray in your hand and you might even consider driving your dog to a different part of town so you can walk, safely.