Today, you can give every family of four in the world a three bedroom house on a 50-foot by 100-foot lot, with a nice front yard and room for a garden in the back, and fit the entire world population into the State of Texas with room to spare.
I think for mankind to be sane, there needs to be enough wilderness to let no home be visible from another without impinging on necessary agricultural land, say a square mile per person.
This is a post industrial world. It runs on fossil fuels and smart tools, we don't need enough people to do everything the hard way like we used to.