PLEASE DO NOT PET A DOG ON TOP OF THE HEAD!
No matter how big the dog is, any hand looks big coming toward their head.
If the dog is a rescue then, most of the time, we do not know what happened to them in the past. If they were abused then they will cower when your hand comes down or they can get aggressive and give you a warning bite, that can still draw blood.
Always put the top of your hand out for the dog to smell you first, then PET FROM UNDER THE CHIN, LIKE A CAT!!!
If the dog is very nervous or shy, try to give them a treat to show them that you will not harm them. Try your best to not stare them directly in the eyes because it is intimidating for a nervous dog.