How much land does it take to support New York City?
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).