As we mentioned earlier, a dog whelping box should be big enough for the mama and pups to comfortably move around. Yet, it should be snug enough so that it retains body heat. The general recommendation is that a whelping box should be one foot longer than your mama dog. Only measure your dog stretched out, as the mama has to be able to move and stretch comfortably around the box while caring for the puppies. Don't forget to consider the height of the side panels, making sure that they're high enough so that the puppies won't manage to get out of the box. By the way, you can also get some hints from websites that sell different dog whelping boxes. Just take a quick peek at what sizes they offer!