The more space they have the better, so 12sqft per duck in run space, excluding the pool would be best, although I feel you could likely get away including it into their run as well.
Instead of focusing on the most minimal space needed for the ducks to be content. Focus rather from a welfare perspective, and look at how much space you can provide in an efficient, and economical way, to allow natural foraging, and behaviors. Space requirements for Calls are often...