Translation:Airports are big.
That would use a different sentence structure: おおきいくうこうです。 (Possibly preceded by それは.)
I assume because a plural is implied but I don't exactly know where or how, so I would appreciate if anyone could fill us in!
I think that's not the case here. Duolingo said the correct answer is "the airport is big". Although, before I also had this sentence and I answered with "airports are big" which is marked correct. Anyway, without the context it could mean An airport is, Airports are, The airport is or The airports are.
は is used of が here. は denotes airplanes in general. が denotes a certain airplane or certain airplanes. This is from my understanding, and correct me if I'm wrong.
You are right about が, but は is a bit complicated. It indicates the topic of a sentence conveying the meaning "as for". So it could be referring to airports in general or some certain airport(s).