i too am doing my own whelping box but not sure of height up from floor and in from sides. Box is for springers any ideas ....CHEERS   FORGOT TO SAY MEASUREMENTS ARE FOR PIG RAILS

I put the pig rails at the height of the bitch's spine when she's lying flat on her side, and about the same distance in - roughly about 4-5 inches for my breed. I've found they only need to be in place for the first few days.
:)