The United States is 103,671,742,065,706 square feet. I'd say that a large percentage of it it is at the very minimum good for maintaining life. Obviously there are swamp lands, deserts, mountains, bodies of water, etc that wouldn't be so good, but let's ignore that.
That gives you 14,560 sq ft per person or about 1/3 of an acre per person. Still not great amazing, but not terrible either if you consider that a family could easily combine their properties to make a larger property and share food/resources. A family of 6 for example would have about 2 acres of land. So I'd say that you could at least somewhat comfortably settle the entire world's population into a country like the United States.
Another option is to move the vast majority of people into cities and use the remaining land (which would be quite a lot if you got people out of suburbs) to raise food. With cities you can stack vertically.