There's a reason that a question like this is so difficult to answer. It's quite normal for a population of pretty much any species of organism to explode in growth when their carrying capacity has been dramatically raised, as ours has by means of technology. What's worrisome isn't that we change the environment, because pretty much all organisms have an effect on the environment, and we have no less right to do so than those other organisms do - but rather that we change it on such a massive scale.
I don't personally think the issue is really one of population size at all. We will eventually reach carrying capacity, and I doubt it'll be when we simply have too few resources to go around. The issue is finding a way to maintain our population while cutting back on our effect on the environment. More likely than not, the answer to that problem will be the same as what allowed our population to grow larger in the first place - more technology. Already we have viable ways to reduce CO2 in the atmosphere. We have viable ways to cut back on manmade climate change, what we need to figure out is how to grow and raise food in a viable way that won't harm the environment, and possibly allow us to restore it. As I said - the issue isn't the size of the population, but how we choose to manage it, and I'm optimistic about our ability to do so.
