One key tenet of ecological economics is that we need to move from a political and economic system focused on the growth of production (GDP) to one focused on constant biophysical throughput. The latter concept is basically an amalgamation of everything humanity takes from the physical world and all the wastes that are returned to it. On the first side of the ledger are withdrawals like ore and hydrocarbons; on the other are wastes including greenhouse gasses and other forms of pollution. While it could not be expressed in the form of a single number, it is fairly easy to imagine a suite of key physical and energy flows through which the aggregate size of human throughput could be summarized.
The basic idea has appeal for several reasons. Most obviously, it addresses the concerns that exist about how much impact humanity can have on the world without causing key biological and physical systems to fail. It also partially addresses the question of how to ensure that human lives become sustainable without becoming unnecessarily unpleasant. It’s the human throughput that actually weighs on the world, not GDP. Even in a situation where the throughput was constant, welfare per person could still increase in many ways: things could become more technologically advanced, better designed, more elegant, etc. They could also be improved significantly by more effectively eliminating situations of needless suffering, as with the treatable diseases that continue to take a terrible toll in the developing world. Of course, per-capita improvements could also be achieved with constant throughput and a falling population.
One objection to the idea is that, when it comes to renewable power, we are nowhere near the physical limits of what is possible. The total quantity of solar, wind, and tidal energy available is momentous, and it doesn’t seem sensible to focus on the total size of human withdrawals from those flows. As such, perhaps the steady-state approach is better suited to non-energy resources, while on the topic of energy, the drive must be from unsustainable forms (oil, gas, coal) to semi-sustainable forms (nuclear fission, etc) and eventually to fully sustainable options like concentrating solar thermal, hydroelectric, and geothermal.
In the end, the prescription for humanity seems to resemble a cheesy grocery store magazine diet: avoid carbon-intensive fuels, manage resource use and waste flows, and feel free to use all the renewable energy and carbon- and resource-neutral technological advancement as you can manage.