
I have a giant breed, I don't think it's the same as yours but is only slightly bigger. I bought a 6 feet by 6 feet box which was the biggest size I could get. It is great for a large litter as they have plenty of room to move around when they are up on their feet BUT it is a nightmare to keep clean as I need so much newspaper and vet beds to cover it. In hindsight, I wish I had opted for a 5 feet by 5 feet or maybe a 6 x 4.
I also purchased the floor which was optional but found it difficult to keep clean as it basically comes in plastic planks so food and other debris (poo !) can get imbedded in the cracks between the planks. The floor is fine when the pups are small but when they grow and start to rearrange the bedding and newspapers, you really need to remove the floor strips daily to clean inbetween them. It is amazing how far a litter of lively puppies can spread their dinner !
Mine didn't have a door, just an opening about 12 inches off the ground but my husband made a door from some extra planks that were included as packing which worked really well.