There is a huge parking garage in Phoenix that is 5 million square feet. I have done a bid for the whole thing, but a different group is responsible for each floor, and I was not able to get them all to work together to let me clean it. Something about being incredibly expensive. Cheap skates.
The largest job I have received on one job order, and it was in various areas was 2,000,000 sq ft. This included parking garages, a tourist area, and a lot of downtown areas for a city, like all the sidewalks in about 2 square miles.