How big should I make my pen?

[cgjsmith]

I was so inspired by ya'lls duck pens i've decided I need to make my own. I have 4 black east indy's and 5 runner ducks. So I want to make sure its big enough. I want them to be able to free range during the day. Is there a guide of sq feet per duck? Thanks Crystal

 

[headred]

I have 7 and my pen is about 10 x 10, see website (scroll past the fish . Mine will soon be free over my backyard, but this space seems good for them.