* Shih Tzus are a breed: Shih Tzus are a specific breed of dog. Being a "big Shih Tzu" is not a recognized breed variation. All Shih Tzus, regardless of size, are the same breed.
* Size doesn't determine fertility: A female dog's size is not a factor in her ability to become pregnant and have puppies. The determining factor is whether she is sexually mature and has been bred with a male dog.
* Breed standards: While there are slight variations in size within any breed, Shih Tzus are generally small dogs.
If you are concerned about your Shih Tzu's health or potential pregnancy, it's best to consult with a veterinarian. They can provide the best advice for your dog's specific needs.