Ducks are great layers, no drake necessary! Let's see: 10'x10'=100 sq feet. From 100, you will want to subtract any square footage that is occupied by stuff, like a house or waterer, or feed platform. Then take what you have remaining, and divide that by... (Please help me out Duck People!)
(Now this part depends on how often they are confined!) All the time, that means that you need much more space per bird than a flock that is only confined at night. Here my pen is 12'x24'= 288 sq feet. I subtract the 2 outdoor houses, the straw house and my water patio, and I am totally at capacity with 12 ducks. And mine range during the not winter seasons. If you have an all female duck flock, I think you should be fine with 6. HTH