If the population density of the United States was equal to that of Brooklyn, the entire US population would fit into New Hampshire.
The state would be ruined, though (imagine a Brooklyn-like sprawl of that size), but the rest of the country would be green and pleasantly devoid of people!
If you used Manhattan's population density, Dense US would shrink to more than half that size, roughly the area of Teton County in Wyoming. Manila, the capital of the Philippines, has the highest population density of any city in the world (111,000 people per square mile)...if the US was that dense, the population would fit into any number of tiny Alaskan islands you've never heard of or a square 52 miles on a side.
