one more thing as well.
Many times mixed breed pups become larger than their partents.
For ex. a friend of ours has a mix of a small Poodle and Silky Terrier but her dog is huge
is the size of a border collie almost. Also know mixes of Pomeranian and Chihuahua who are bigger than both parents. And much more.
So even though the puppies might be born small they might grow big.
beside that I agree with most people here, your girl sounds like she is too small and there are so many risks with breeding her, you must be ready to lose her in the progress before going on with something like this. Also I do not agree with breeding mixed breeds. There are plenty of homeless/unwanted mixed breed pups and bringing more mixed pups into the world will not help.
I would rather incurage your friends, that want a pup from your girl, to go to a shelter/rescue and save a puppy there
Save a life there