Pregnancy by a larger breed

Question
I have a 14 month old female shitzu, she was in her 2 heat and managed to get through the fence to the dog next door. He is a 10 month old 80 lb. boxer. I'm not sure if they mated or not but am nervous they may have, would puppies of this size breed kill my dog if she was to give birth. I plan on taking her to the vet to be tested, but I just was wondering the answer to this question? thank you

Answer
When you have a female in heat you need to have her supervised
or kenneled at all times especially if you know there are any males that are un-neutered in the area.  Please take your female to the vet immediately and have her spayed.  And to answer the question if you were to get pregnant by an elephant would you be able to survive it?