I think i read around here that it's 10' for each duck. (5x5???)
For my own reference, I am going by the dimensions 1lpoock stated when he made his aviary.
His is 12' x 30' and holds 4 pairs of ducks.
The one i plan to build is about half that, but i will just be starting off with 1 pair. It looks like i would probably be able to have no problem with 2 pairs of ducks, though.
Also, it may depend on the type of duck you plan to get.
I am planning to get Hottentot Teals, which are very small ducks.
What kind are you trying to calculate dimensions for?