Unfortunately, I'm not really sure this is something that can be easily answered.
All dogs are unique not only in their natural behavioral traits but because of how they have been trained, raised, and the experiences they've had. It really depends on what's going on with the individual dogs.
If you're worried about sitting for untrained or aggressive dogs (a totally legitimate concern), make sure you do double duty when chatting and meeting with prospective customer and their dogs. Ask up front if their dog has any behavior issues, then assess the dog during the meet and greet.
Always make sure you feel comfortable with the idea of caring for the dog. Always know what your limitations are and only take on clients that you feel comfortable with.
Namasté, I KEEP TELLING ROVER TO MAKE A OPTION TO LET SITTERS CHARGE DIFFERENT RATES FOR EACH SIZE DOGS! Is ANNOYING TO HAVE TO LOOSE CLIENTS OVER OWNERS THINKING THE RATES ARE TOO HIGH FOR THEIR DOG SIZE! TELL ROVER SO THIS CHANGES! Namasté, Marcus
agreed! keeping a Mastiff is a whole different job than keeping a terrier