I agree too small for that many chickens ... 3-4 maybe ...
I also agree with leave door to run open, or make hardware cloth screen door to replace it, and leave the factory vents open all the way ...
While waiting for pictures and measurements ...
Chickens poop ALOT at night ... chicken poop has ALOT of moisture in it ...
Also chickens breathing put out quite a bit of moisture too ...
While they only sleep in it may apply ... they are cooped up in there for more than a third of the whole...