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.
Actually, this got me thinking: Were there ever any cities of comparable size that were entirely landlocked and supported purely by their surroundings (before, say, the Renaissance, or even the industrialization)?
@racefortheironthrone: Your answer seems to imply there were some, but I can’t think of any. What have I missed?
Well, there is a middle ground between a port city and being landlocked – being on a river (think the city of York, on the confluence of the Ouse and the Foss) or on a major overland trade route (Florence).
But yes, there were landlocked cities although they tended to skew somewhat smaller than port or riverrine cities: for example, landlocked Madrid was 30,000 strong in 1561, whereas Paris was more than 150,000 strong by that time. Similarly, Vienna in 1500 had 45,000 residents, while London had about 100,000 at that time.
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