Well, back when i was a kid, i thought the Nostromo was that huge ass tower thing lol, back then i thought it was self maintenanced and that the crew was only to keep tabs on it ect. But years later i found out the Nostromo is actually the small craft they landed on the LV-426 with.
I think it should be more than one deck large though. Bret and Parker were down on engineering. and if Aliens Isolation had anything to go by. The Ladders we see in the movie clearly are used for going into maintenance ect. I think the primary area though is small.
Even Isolation used the same layout so we know the exact size of one of the decks. There's probably 3 decks at the most not including the air shafts ect that are used throught.