The estimates of how much space each human will have are pathetic. They firstly under estimate how many humans have ever lived. They suggest the earth will be changed to make just about everywhere habitabal, but even so... they do not take into account the "footprint" required to sustain one individual... Let me explain.
You take up a lot more room than your apartment or mansion. The oxygen you breath requires a certain amount of growing space (several acres per person per year) to sustain you with oxygen. You don't own this space but it has to be "out there". Then there is the agricultural space required to supply the x number of tonnes of food you eat per year. The same could be applied to the water you drink. We are struggling to sustain the worlds water requirements even at today's population levels. Then on the other hand you have your waste disposal requirements, not to mention sewage. (you could dump all this over the neighbours fence I guess). Finally there's the 6.6 tons of Carbon produced per person per year -
when you ask these questions, you start to look at what is the absolute maximum population the earth can sustain - even with the deserts turned into pasture. It is not simply a matter of working out landmass divided by an estimate of the total number of humans = "living space".