From talking to toy breeders at the shows I go to, and they typically won't breed anything under 4lbs. Alot of times, even 4lbs can be a c-section. You have to understand that, sometimes weight isn't the biggest factor. You have to also evaluate the size of your bitches private area(sorry no spell check and horrible at spelling). If she is very refined in the hips area, she could have a difficult time passing puppies.