Holderread recommends 25 square feet per duck if that's the only space they have, 10 square feet if they also have an area they can forage in aside from the pen.
About 4 to 6 square feet in the shelter, depending on the size of the duck and the climate - up here, there were days the Runners had to spend 23 hours a day in the shelter. So I want them to have lots of elbow room. Do ducks have elbows?
Yes, that's what they smack each other with when everyone wants to be in the tub at the same time!
As for how much space -- I would use the minimums as just that, absolute minimums -- look at ideal, and then give them every inch you possibly can. The more space they have, the happier and healthier they will be and the happier you will be. Cleaning up after crowded ducks is a full time job. I don't recommend it. Plus there's the whole poultry math problem, which you are apparently experiencing now -- the 'I'll just get a few more....' .....and pretty soon you're overrun.... It happens to the best of us!