How much land does it take to support New York City? New York City has about 8.5 million people living in an area of 300 square miles which gives a population density of 28,300 people per square mile.  The food required to support such a high population density is mostly grown outside of that 300 square mile area.  Likewise, the fresh water required to support that population is not collected within that area. 

How much land outside of NYC is effectively dedicated to supporting it?  More generally, how much reasonably productive land does it take to feed and hydrate a human being on average?
The average ecological footprint of a US citizen has been estimated as 8.00 global ha.

Multiplying 8.5 million by 8.00 hectare gives 680,000 km², or around 5 times the area of New York state.

Of course, this calculation contains a lot of assumptions and simplifications.  This doesn't mean you could simply allocate 680,000 km² around New York City and use this to support the lifestyle of their inhabitants.  There is a considerable uncertainly around the calculation, and the land is spread all over the world (New Yorkers want their bananas, almonds, and iPhones too, even if none of those are local).