Well, there are pups that are stillborn or born premature, then you have runts...which are usually small or larger often called giant 'runts'.
As for the father many large breeds could fit the bill such as Bernese Mountain dogs, Rottweilers, German Shepherds, Newfoundlands, Great Pyrenees, Tibetan Mastiffs, American Bulldogs, Great Danes, Irish Wolfhounds, Akita Inus, Dobermans, Wolfdogs which are a mix between a wolf and a dog such as a husky, malamute or shepherd. Bloodhounds can be big too, there's also St. Bernards and the South African Mastiff.
In one of my story ideas, one of my dog characters is a mix of various breeds and wild dogs such as the Rottweiler, Basenji, Red Wolf, Border Collie and others, making him a large wolf like dog.
I would suggest doing research on mixed breed dogs and purebreds.
If you do publish your book, I recommend posting it on webook.com for feedback.
I'm curious about how your story will turn out