It is an often ignored fact that everything has its limits. The maximum size of the human population for this planet is just one of these, but, what the population does has a far more immediate impact on ecology than the number of people doing it.
As I see it, almost none of the population has any real idea of the environmental impact caused by simply living their daily lives. People seldom look at their footprints and nearly always disregard what they have trodden on. So, if some enlightened government made that disregarded impact far less damaging, then people could go on living very much the way they are and the ecology of the planet could recover.
The only global consensus that would be required is for a common understanding that there are problems with limits - and that taking someone else's resources is not acceptable. The need for trade itself will be greatly reduced by the massively reduced need for resources enabled by being Green. The ending of planned obsolescence, the introduction of a repair and rebuild culture and a general lowering of demand for new things being replaced by a demand for new or better experiences are just some of the innumerable methods that shall have to be used globally and locally, as appropriate to local conditions.
Global trade is important for the quality of life, far less important is the quantity of stuff in that life. Trade should bring novelty not 'coals to Newcastle'. The quantity of exports and GDP etc has nothing to do with real lives other than via money. We can cut out all of that and the associated trade sanctions by making the reasons for them irrelevant. For example, using renewable power would kill off the oil barons / countries / industries - not the people who work for them as by then there would be enough stuff properly distributed to go round.
It is not the job of all Green Party members to have all the answers. The odd person may have a partial answer, and when enabled by a Green government they may find the rest of an answer from someone else - no mater who that person votes for. A Green government should not claim to have all the answers either - but should empower people to find answers by asking the right questions - ensuring the search is going in the right direction.
Slowly, as people begin to feel safe, and as the level of education - for blue sky thinking or just personal interest and personal growth increases, the birth rate falls. This is an established interaction seen all over the world, there is no reason to think it will fail now. Thus, population first levels off - then falls.
There are problems with this picture - firstly that it all takes too long and resource wars break out before people learn how to sew or run their 3d printers. Solution - go in to space - mine stuff up there - have your wars up their if you really must. Climb to the Moon and Mars rather than invade some other part of the Earth.
Secondly, the powers that be want to keep being in power and so continue to stop a green surge pushing them out. The solution there is already happening. The powers that be are powers partly because they are lead by intelligent ( hidden ) thinkers who are totally aware of the planets limit. They have been working and planning for almost as long as Greens for what to do in the future. They have been waiting until the last moment to do it so as to get the maximum value from their infrastructures - but now they know that time has run out and they are changing. Possibly they judge that right time to go Green by public reaction, whatever the reason - it is happening. Cars are being built that run on renewable energy, Oil companies are greening their images, China is advancing solar cell technology, US residents are making algae grown in their back yards power their homes. It's all happening folks !
None of the above is either utopian or outside our current technological ability. It's all a matter of the social will - and this week's massive membership rise further supports me in thinking that having my kids was a great idea - they are both Green Party members.