I think it was Sheena who wanted to know about whelping boxes, but the thread seems to have vanished.

A whelping box needs to be big enough to allow the bitch to lie flat on her side with her head down and her legs straight without touching the walls, which for most breeds need be no more than a foot high. The removable pigrail should be at the height of the bitch's spine when she's lying flat like this (basically half the width of the bitch).

Hope this helps.
:)