On the contrary, smaller countries are less effective in the international arena (militarily, economically, and politically). Banding together with other countries only creates a meta-republic of countries (e.g. the EU).

So I think the original question is quite valid. There's a sweet spot somewhere between "as big as possible" (e.g. US or Russia) and "as small as possible" (e.g. city-states).

My gut would be that it's more determined by economic production and ethnic subgroups than some constant. Sufficient economic production to play competitively on the world stage, while grouping as many like or agreeing ethnic groups as possible (and preventing their repressing minority groups).

Or in other words, international jerrymandering.