What would be a 'good' size for the future total human population of the Earth? 
 NYT report on Mobile elements reveal small population size in the ancient ancestors of Homo sapiens, doi: 10.1073/pnas.0909000107
 Here I quoted six million as the probable number at the end of the Paleolithic (approx. 10,000 years ago) and five hundred thousand if the abundance of humans were linked simply to their size according to a curve of mammal abundance from mouse to whale. I don't recall where the first of these two figures came from.
Photo: Congo gold mine by Marcus Bleasdale