It's an interesting way to grasp the enormity of large numbers (billions) but 1 person per square meter isn't habitable. NYC is considered very crowded with 1 person per 96 square meters if I did my math right (27,000 people per square mile).

To fit the world population into a crowded mega city as dense as NYC it would need to be bigger than Texas (277,000 square miles), which is logistical nightmare to say the least.

If you take an urban sprawl city like Phoenix, with a density of 2800 people per square mile you'd need a megacity almost the size of Australia (2.7 million square miles). More realistic, but still quite challenging to run a city that size.