I guess call ducks would possibly be ok, but personally I wouldn't due to space for wing spreading etc. they are more active than people give them credit for when housed.
I have 2 Saxony ducks and they are currently in a shed 8 x 6 at night or shed + aviary 8 x 16 shared with the chickens, depending on where they put themselves to bed
I think if they are small, as debsandpets says, then it should be ok if they have a larger area in the day. They will definitely need access to fresh water they can get their heads into while in the Eglu pen though. You could use a large clip on container.
Call ducks are great pets btw, especially if you hatch them yourself!
The Eglu itself is great for sucks as long as they arent too tall or wide - obviously you take out the roosting bars
The run however does have enough useable space for anything bigger than call ducks
A couple of call drakes might be nice if you dont mind getting no eggs, my males are great characters and virtually silent
An alternative would be to have an eglu in a larger more squared off run, my ducks live in a plastic hooded cat litter tray in a 6x6x3ft tall run
Ducks dont tend to use coops in the way chickens do, mine only lay in their cat litter tray they sleep outdoors so a secure run with small mesh is essential