Space is a social lubricant. More is almost always better, budget permitting.
I'm going to assume you plan to get either all hens, or only a single drake (in which case, I feel badly for the hens - drakes are randy and pretty brutal). Plan a minimum of 6 sq ft per bird floor area (and that's all you get, floor area, generally no "layered" space, like with chickens) in the coop/house and at least 15 sq ft per bird for the run, more if you can manage it.
Adjust according to climate - you didn't list a location, so its impossible to know if you have seasonal weather concerns that might keep them in the house for extended periods (in which case, more space!) or if you have generally mild weather which would afford you the option of a combined coop/run more like 18-20 sq ft/bird total.
Now, take the above with two grains of salt - I'm basing the recommend on my Pekin Ducks, a larger breed than Khaki Campbells, which I've never owned. Those are the minimum recommends for my breed, and I'm in a forgiving climate, but seeing the way multiple drakes behave together, or top female behaves with the other females when her position is threatened, I'm glad I gave them even more space than that - and am currently forced to keep my ducks as two separate flocks, as top girl and solo drake are both picking on my biggest girl of a recent hatching, though she appears full grown. (She'll get bigger, she's only about 4 months old currently - but she won't look a lot bigger, they carry their weight deceptively well as they bulk up)
