Ideal Population Size

The USA accounts for 1.9% of global landmass, and therefore 1.9% is a first approximation of the ideal proportion of humanity that should live within the boundaries of USA in a future sustainable global human society. 

Estimates by Limits to Growth advocates of the ideal global society are in the range 500 million to 1 Billion.

That would give us our first approximation USA number as 10-20 million.

Now you could argue that not all land mass is the same, some are more capable of supporting and even thriving despite human existence due to better climates, more temperate areas, more atmospheric moisture, whatever.  I don't actually believe this is true of USA, which has a lot of deserts, etc. (though those could be useful, under limitations, as solar farms).  Current population density is far higher in UK and Western Europe, and many other places.

But even if it were true, that the USA has tremendous geographical advantages for sustainable human sustenance, it would probably only boost the optimal population 2 fold  to 40 million or at very most 5 fold to 100 million.

Now one argument has been made that such numbers are undesirable because "technology as we know it cannot be sustained."  This is wrong because first we have to consider the global numbers because technological development is global.  Secondly 500 million to 1 billion is a better number for a technological society to make real progress, in a sustainable society, as compared to 8 billion where most resources ought to be devoted to the mere initiation of a drive toward building a sustainable society, that being hardly imagined amidst the imperialism, wasteful materialism, and profit maximizing we have now.

Given that material wealth is ultimately constrained by resources and technology, the wealth level should be universal.  We should all be engineers, artists, doctors, or scientists, with technology doing all the energy expenditure.

Others argue that reduction in consumption could save us.  But no possible human society is devoid of waste...and waste can even be worse for very low consuming human societies may lay more waste because they have no other choice.

Meanwhile, NYTimes today had editorial decrying the drop in US population grown rate (now 7%) and exhorting us to remember we need higher growth to continue to dominate China.  Earth be damned apparently.

I'll leave it as exercise to reader to determine degrowth rates to reach, say, 100 million in 2100.  And compare what is required if we start now or in 2080.