Your English word-order is wrong. "A large mirror is hanging on the wall." A general statement of a possible rule is that phrases and even clauses which act as adverbs of location usually come after the verb, and subject comes before verbs in English declaratory statements. So, "A large mirror (subject) is hanging (verb) on the wall (adverbial phrase of location)".
Sometimes, you can get away with the location being stated first, but you're more likely to get it right if location comes after the verb.
I'm not a moderator - so I can't tell what their a/the rules are. That said, this particular Russian sentence sounds to me as a fairly abstract and neutral statement. A natural context would be a general description of a particular room or wall. Hence "a mirror" would be the natural choice here.
On the other hand, if you were to tell someone about a location of a mirror, which has already been mentioned, you would change the word order and say "Большое зеркало висит на стене". This draws the listener's attention to the mirror (as opposed to the wall, as in the original sentence). Personally I would translate this second sentence using "the mirror": The large mirror is/hangs on the wall.