I find the 81″ length by 39.25″ width by 19.75″ height balanced, offering enough height for easy access.
Item Tag: USD06-174V00CF0145
Wooden Whelping Box for Dogs for All Sized Dogs, 81″ x 39″ x 20″
A nice touch is how the pet playpen folds flat, so it tucks away between litters.
