Metrolina Greenhouses is the largest single-site heated greenhouse in the United States at 162 acres under roof. That's over one-fourth of a square mile (that is like "a half-mile by a half-mile" or 160 football fields side-by-side).
They do not sell direct to the public, so you may not have heard about their operation. If there are any doubts about the future of large indoor growing operations, take a look at the "mega greenhouse operations" already in existence.
The photo above shows racks-and-racks of plants and rows-and-rows of ferns hanging from the high ceiling of the greenhouse. Even with 160+ acres of greenhouse space, they make use of the vertical space.
How do production levels of this sort of "mega greenhouse" compare with "traditional" outdoor farms of equivalent size? (Where is the data?)