My two cents; Is a smaller, medium or large girl; are just as likely to have no problems or problems delivering. However it's the litter size that will make the difference on how big the pups are to get; you get a small litter the pups can grow bigger inside the dam; your more likely to have problems with delivery. You get a big litter the pups will be smaller. Goldfish in a bowl scenario; a fish grows to what it's container allows, and Nature always seems to take care of a lot on it's own, despite what we do to mess with it. I wouldn't over-worry about breeding a smaller female to any-size male; anymore than I would worry about breeding my female period. Pregnancy is always a risk, and all you can do is just hope the outcome is worth it.