I wonder what the optimal population is for modern US quality of life. Clearly at 3000 or so, we wouldn't have enough specialization to have a decent quality of life. At 10b (and even at 7b), we're stretching available resources. I do think 1-2b would be a good number (if optimally selected).

Arguably if we had better technology, even an increase over current population could be an improvement (assuming a marginal extra person was engaged in productive activity); most of the constrained resources seem like they could be dealt with using cheaper energy (making recycling/low grade ores/etc. recoverable, better transportation to reduce crowding, dealing with pollution, etc.)