According to Google, the size of Dallas / Fort-worth metroplex area is 9,286 square miles. That is 28,764,444,444 yards / 7 billion people = each person gets 4 square yards.
So while a 6'x6' space isn't a lot, you could fit the whole population of the earth in just one area with plenty of room around each person.
I think politics and greed are a bigger cause of starvation and pollution than population size.
The question suggests that you expect the answer wouldn't simply be that there are counter-forces to politics and greed that are at least strong enough to make a difference. That doesn't mean that said forces don't need to overcome politics and greed in order to make starvation less common.
The maximim number is a multiplication of the number of human by their average CO2 impact, and currently it's too high. The average carbon footprint per person is 7 tonnes CO2e per year. 2T might be sustainable, and achievable (with smaller vehicles and less consumerist behaviors)
No one decides about it for now, like we do for other invasive species, but we should probably start considering regulating ourselves
Even though the article is about biomass and not consumption per se, I take it like a reminder the global human population (in our current state) is currently throwing the atmosphere's equilibrium off into a negative feedback loop that may render the planet uninhabitable for many - human activity has to change, otherwise it will inevitably cost (more) human lives.
Err, ok? What's the 'correct' number of humans on this planet, and who decides who gets to live and reproduce?