Based on curve fits of happiness and life expectancy to global ecological footprint, total happiness would be achieved with 47 global hectares per person, and 100 years of life expectancy could be achieved with 67 global hectares per person. If all bio-productive land were used by people (none available for other species), Earth could sustain 239 million totally happy people or 168 million people living to 100 years old. If we left 10 percent of bio-productive land for other species (a purely arbitrary number at this point), then under ideal conditions the Earth could support 150 million people (half the current U.S. population).
Based on this and the previous analysis, we can say that under ideal conditions, a population between half and one times the current size of the U.S. could be sustained within the natural capacity of the planet. The GDP per capita would be between 13 and 27 times the U.S. value in 2000 (where the latter number corresponds to 67 global hectares per person).