So very sorry for your loss. Lymphoma is evil.
I don't think yorkies become ill "just" due to size. It is a lot about breeding as Taylor said. If I were looking for a breeder I would ONLY get a puppy from one who did the OFA testing for known genetic conditions. Going to one, you will definitely pay more, but getting a well bred yorkie puppy is best so that some of the common genetic issues are not something you will deal with.
Look at the link Taylor shared. You can see all of the conditions in yorkies that CAN be tested.
Best wishes in your search.