There is still plenty of room left, and theoretically there are enough resources, although they are not shared out in a fair way. I doubt we'll ever need to implement such measures, as nature has a way of kicking the s**t out of life when it needs to. Some new plague, or a different strain of an existing disease will cull huge parts of the world, and it will generally be in the poorer places with poor infrastructure.

Also, who sets the numbers for what reasonable population is? And who decides which people are allowed to have children or not? It's impossible to regulate.