What about Sweden? I think every modern society is able to "operate on a long term sustainable basis and implement a sufficient level of social justice and equality". That is only a matter of freewill and a bit a matter of fairness - since it makes no sense to try hard if your neighbors burn all the carbon you left on purpose. That was the reason why I spooke about a contract between societies... Without fairness we will be doomed until someone suceeds us with a new deal/contract.
I think Sweden fails to "operate on a long term sustainable basis and implement a sufficient level of social justice and equality". They do have a good reputation for social justice (recent riots notwithstanding, I suppose) but a quick check on their carbon dioxide emissions per capita shows they aren't remotely sustainable.
I agree that you're right that social justice would need to exist between and within societies but not entirely that it makes no sense to try hard - as other nations will use the carbon anyway. This might feel correct as a short term justification but there are good reasons why one should still try hard:
1. The nations continuing to use carbon based fuels are in effect addicts pursuing a short term fix, even ignoring climate change, they must go through a withdrawal of some sort as peak oil bites (these are finite resources)
2. A society that put itself on a truly long term sustainable footing - long term - should have an advantage over all those that have not that will continually be meeting sustainability related issues of all sorts (until they emulate a society that has put long term solutions in place)
To be more explicite on your point: The maximum size is not limited by sustainability but rather by your equality condition (as I explained in the prior post). A sustainable society could actually be larger, because they need much less resources.
I'm not sure this follows. I would question the definition of sustainability at this point. Resource use per capita in a sustainable society might be smaller - but as soon as you say that you can make the society larger as a result - you mitigate out all your gains (as is tending to happen with energy efficiency, especially in vehicles).
Furthermore I question that sustainability must not also include some notion of the gross physical footprint of a society upon the planet. It isn't sustainable if it involves consuming all the available land area of the planet for crops, habitation, etc.
Hence population is an issue regardless, and on those two counts you can't grow it more sustainably (not within the bounds of the earth system at least).
Likewise I don't believe you can ever take a settlement (eg a city) in isolation and hold them up as a model for sustainability - as a city there is a large footprint that extends far beyond the city (ie agricultural practices, resource mining, transport, etc).
And the living is really nice there. If you look in the poeples faces there, quality of live is surely increased by sustainability. Perhaps due to the fact that sustainability puts an end to the fight between generations?
Speaking only for the UK - but my feeling is that the intergenerational issues are not to do with sustainability, but to do with wealth. Older people can be debt free, own their homes, live comfortably, afford to eat and fuel their vehicle, have a university education, final salary pension, good healthcare, etc.
Younger people... well, TerryM said it better than I could - we get jellyfish.