As a good rough first approximation of what a real, moral and rational population-law would look like, I submit that we as a species should not use more than 1% of the least available living resource (rainfall, freshwater, aquifer, arable land, CO2-fixing, whatever) (LALR) that we use on this planet.
Obviously we will use more than that in our industrial and urban cores and surrounding suburban countours, but all that should be counterbalanced by law by preserve elsewhere.
Also, land which could be connecting preserves and preserves to significant landforms (river, coast) should be counted as somewhat more valuable as preserve and in terms of preserve needed to counterbalance.
And some lands and landforms should of course be given enhanced protection (heads of watersheds, karst, forest, habitats of significant endangered species) and counted as even more valuable in terms of preserve needed to counterbalance (if other than grandfathered/conservation-easement-overlaid development/habitation is to be allowed at all).
Also, the issues of CO2- and heat-generation should be considered to see whether they represent significant enough population-pressures on our planet to be counted along with or even before the foregoing (I believe simple heat of population and its effect on rainfall is a direct cause of many of Florida's, California's and Australia's problems, and no doubt elsewhere, including throughout history).
The foregoing represent the sort of constraints on our numbers I submit a real, moral and rational population law would be based on.
Obviously, our population at present undoubtedly far exceeds anything imaginable under such law.
I do however believe, unlike, apparently, some "Greens", that we as a species have just as much right to live as any other, and as a technological species.
A one-child-per-couple law is the least-onerous of all transitional laws to get (ever more) slowly to where we need to go, with more allowed where that would cause vanishings of subpopulations, and also where voluntary restraint causes population to actually dip below the desired level, such extra children allotted by lottery and raffle, and perhaps a very small number by auction (proceeds of raffle and auction to be split between social and green budgets).
(See also my posts "The Overpopulation Equation"
and Photonomics: The Photosynthetic Economy
Keywords: conservation, ecology, law, overpopulation, population, sustainability