You can keep a trio of buttons in a 10 gallon fish tank, so you would need at least twice that. I've seen a lot of people use the plastic bottomed rabbit/guinea pig cages for them. You can find them on craigslist fairly cheap.
Would the ducks be in the pen during the day or at night? If it's at night, they would basically need a dog house to sleep in, with a door to lock them up. Other than that, I think they need about 8 or 10 sq ft per bird, though I'm not sure about that.