I would stay both: rate of change (up or down) and overall size of population - there has to be a ceiling somewhere - if you start filling your bucket with water even slowly at one point it will start overflowing.
There has to be some sort of ceiling, sure, but the point of focusing on a rate of change is that the ceiling is constantly moving too, no matter what you consider the "ceiling".
There are many ceilings. E.g. there's the carrying capacity of earth. But that carrying capacity changes with technological improvements. There's the carrying capacity of the solar system that changes as we get the technology to settle more and more other planets. There's the number of housing units currently available, and the amount of food currently produced, and so on.
But a lot of these current ceilings are highly malleable given time to address them, which is why what matters is the rate of change.
Even if one assumes there is a hard ceiling on capacity for earth that no technological advance can overcome, as long as the rate of change is low enough we can bypass that by building off-earth colonies fast enough .
So ultimately, while you're right there are ceilings, it's still the rate of change that matters: We can build or obtain more buckets to move water to, but we need enough time to do so.
