Tl;dr, fetal size is regulated by the mother, so the puppy would be Chihuahua-sized.

It also seems this mix hasn’t been done as a formal experiment (internet anecdotes don’t count), but something similar has been done with draft horses and Shetland ponies; crossed foals born from pony moms were pony-sized, and draft-sized when born from draft moms.

And now you know!