@clintflint - There are quite a few breeds that have been tiny for generations and are usually perfectly healthy. And at least when you get a dog from a breeder, you know how large they are likely to get.
If you are looking for a puppy that will be a small adult, you just can't trust a dog you get from the shelter. And that's not a matter of being cruel. In fact, it's the opposite. If you don't have the space for a big dog, then you are just going to end up having to give it away again if you end up with the wrong kind.
It happens with people who buy so-called tea-cup piglets all the time. They don't realize that the full grown pig is going to eventually weigh more than they do.
If you get a proper teacup toy dog breed you won't ever have to worry about that.