The other nice thing about having an "extra" whelping box/area is that it makes clean up easier- just transfer litter to other area while you clean up the first. Depending on door location, you can either do one on each end or two down one side on each end. The box should be a minimum of 4 ft on one side. That would leave you with a 3 ft walkway if you went with the side wall option. if going with the end wall option, have a 1 ft. high divider in place to make the whelping area smaller for the first 2- 2 1/2 wks. Once the pups get mobile, open it up for a potty area before letting them go outside (assuming that there will be doggy doors to outside area). They should be fine with access to outside by around 4 - 4 1/2 weeks depending on the weather and as long as they are navigating the door easily.
Along with a sink, you will want a bench with storage underneath for crates, buckets, and other whelping supplies, as well as food. I would carpet a portion of the bench/table for stacking/evaluating the pups, giving meds, trimming nails, exams, grooming, photos, etc… You will want to be able to throughly clean so something very "clean" without a lot of ridges or bumps as far as walls and floors. Keep all electrical outlets at least four ft high, I would actually put mine in the ceiling. Ceiling fans would be good.