I have two night shelters.
One is a 32 sf house with an attached 32 sf porch, (lots of metal hardware cloth), and the other is 65 sf for the main part and another 15 or so added on - 80 sf.
This is comfy for nine runners and two buffs. They are either in one or the other, I don't split the flock at night.
Rules of thumb are that you could go smaller, maybe 3 sf per duck, but I have seen with my flock that there are extenuating circumstances that come up and having the extra space has really helped.
Also, being able to get into the duck house with the ducks has been quite handy from time to time.