This website gives some dimensions for different breeds
Whelping Boxes For Dogs - "How To" Guide For Making Or Buying One.
And the lab one shows great pics of a safety rail. We always had a safety rail to prevent squashing. Large breeds are prone to do that accidentally
LAB TAILS: Whelping Box: The Reason for Rails
And you might like this site, because it has some info on raising pups and such
How to Build a Whelping Box for Your Dog
Not sure as to wether you can see the rail made from plumbers piping in the attachment pic, it is plastic, so easy to keep clean and was cheap to buy and you can pull it apart and store. Sorry about the pic only a phone pic of a few years ago