I think it comes down also to the breeds/size of your Ducks.
Assuming they are nonflying, you can get away with around a three foot fence I believe. The chunky pekins I would imagine you could get away with even smaller.
I agree with Jay, my uses of fences are in large part to keep things out of my animal pens moreso than keeping the animals in. I have a lot of predator pressure in my area and that is something that always is in the back of my mind. That is why my chicken run/coop is built to be the avian Alcatraz but with nice furnishings