How much land would huge cities like Rome or Constantinople or Athens or the free cities in ASOIAF need to feed the population of just that urban city?
Great question! The answer is: large cities in any period of history, be it during classical antiquity, the Middle Ages, the Renaissance, well into the Industrial Revolution, require a very large hinterland to provide the necessary food to feed their populations.
On a general rule of thumb, you’d generally expect cities of those size to command the resources of the countryside around them for at least several day’s travel distance from the city (basically, as far back as you can reasonably get goods to market before they go bad).
However, all of the cities you mention are port cities, which changes the story somewhat: Rome drew its food supply not just from Italy but also from western North Africa etc., Constantinople drew its food supply both from nearby Anatolia and Thrace but also from Egypt, and the Free Cities can draw their food both from their hinterlands but also from Westeros or other parts of Essos.
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