Storey's Guide recommends for medium to large ducks, 2.5 to 4 sf per duck, or if they'll be confined for days during the depth of winter, 6 to 8 sf per duck. Smaller breeds need about half of that.
So at a minimum for runners I reckoned 2 sf each. That gives me enough summertime space for 15 ducks, as their house is about 30 sf and that does not include the veranda - a porch attached to the house, which is also about 30 sf. For winter if I did not get them out as much as possible (which is what I do), I would only have room for about 8 runners.
In your case, 2 sf would probably work. I know someone who keeps her runners in tighter quarters than that, but we have different approaches. I have read a number of reports that the more closely animals are packed together long term, the more behavioral and health problems crop up.
By the way, the house and veranda are their Little Fort Knox - nighttime shelter. For daytime, unsupervised, they have another 160 sf of pen, and when I'm nearby they have about another 250 to 300 sf of garden to run around in, then there are the daily walks that range over half an acre or so - supervised.