Farkel: "then, please tell me, what are your limits on population?"
A little background information before I answer that question.
I believe that self reliant communities of approximately 500 people each. I would model the communities after Arcosanti, an unfinished ecovillage 70 miles north of Phoenix. Land would be redistributed, let's say four square kilometers per community, which comes out to two acres per person. I believe that this amount of land is more than enough to provide several times the amount of food necessary.
If the amount of grain that is being grown is four times the needs of each individual, that surplus will be sealed in Nitrogen packed cans. Presently this technology (which caters to the survivalist movement) is capable of storing dry grains for 20-30 years at room temperature. This would be enough food to counteract the effects of any megadroughts or crop damage of any sorts whether due to disease or climate change. That plus water collection systems in each community would guarantee survival and stabilize the population making it immune to whims of nature (and in my opinion man induced climate disruption).
That explains, in simple terms, how I would prevent the population crashes. Preventing the population from rising (which inevitably perpetuates the cycles of boom and bust with all the human misery that Malthus correctly predicted) is a more subtle matter. In my opinion the inhabitants of these self reliant villages will know unconsciously and consciously that they are reaching a limit as their population begins to rise to 600, 800, 1200 etc.. What they will perceive is that their standard of living is going down slowly but surely. What they will do (the females mostly) is what many are already doing in Russia, Italy and other nations in Europe, even the USA which would be in negative population growth if it weren't for immigrants (the legal kind let alone illegals).
It is important to note that the small size of these communities makes them more transparent and comprehensible. It's inhabitants would understand their "standard of living" would be declining thus creating a choice for them: Quality or Quantity? (Monty Python's "Every sperm is sacred" is a humorous analogy to what I'm expressing). Also, while any individual in this civilization is free to move anywhere he wants, he/she would have to be accepted by the village (which probably would put an end to imigration the moment they perceive that they have enough people.
Also not all of the land would be given up for human habitation. About half or so, in the USA, would be dedicated to pure wilderness without human inhabitation of any sort. Furthermore since the cultural traditions of this village civilization will include knowledge of the past (those who forget the past are condemned to repeat it) the population will be self regulating. No forced abortions, etc.
With a stable population on Earth the only expansion of Humanity would occur after several centuries when space colony technology becomes feasible. The same principles would apply to our Solar System as well as the Galaxy. No mindless uncontrolled exponential growth but population stability punctuated with brief spurts of growth. The "standard of living" then would probably be one "village" or a few residing in an O'Neill cylinder (let's say five miles wide by twenty miles long).
Oh, now that I gave you some background information, let me say, or rather guesstimate, that the "limits" based on the above would be about half a billion to one billion in the North American continent. Yes, you can squeeze a lot more but why bother? What is this fetish with quantity?