Hi Shelley,
I was just dogsitting my friend's pitbull over the weekend
I think the main thing is what you are doing, introducing them at a "neutral" territory. Be sure to praise Zuzi a lot when she show's a positive interest for the puppy.
I don;t think dogs understand size...at least not yorkies (or not MY yorkie...hahaha) They are extremely jealous of their humans so make sure you give her extra loving so she doesn;t feel threatened by the new puppy...