How large do you think a dragon reservoir would have to be?
I would think that the grounds would be large enough for the Dragons to run, fly, play, and burn things with their awesome fire breath. Probably larger than Hogwarts' grounds. In addition to the Dragons, I think there would be a place for the personnel, like dormitories or other buildings.
How would they conceal it from Muggles?
Now, I found this in the Lexicon: " There are presumably dragon reservations all over the world, hidden in the mountains where Muggles are unlikely to come across them." I think the employees would enchant the buildings as well, just for extra secrecy.