It depends on the population in question... and for starters I'm guessing we're talking about humans XD.
Ideally, a population stable at it's carrying capacity is the best. Fortunately/unfortunately for us, we don't really know what the earth's carrying capacity for us humans is quite yet. But anyhoo, if we were underpopulated, it'd be best to grow; overpouplated would be best to shrink; just right would be best to stay where we are. This applies for the global community or smaller stuff like countries. Yay for learning stuff in bio >.<