Can a large male dog get a small female dog pregnant?

It depends on what proportions you're talking about. If you're talking male Great Dane and female Chihuahua then definitely not, as the female is simply too small to be mounted by the male. Artificial insemination is always an option, and required for survival of certain breeds similar to and including the French Bulldog. Technically speaking, any two breeds of dog can mate, even if it requires help from humans.