You can go to about 2cc and still have a reliable, working engine (see model engines) but it would be much use on anything with a wing span of over a metre.
The problem isn't with the air molecules but with the integrity of the internal components. The lighter they are, the more likely to break. At the other end of the scale, if they are heavier it will sap engine power and require a larger displacement just to turn over or idle.
I'm sure you could go smaller if you had the technology or inclination but what would be the point? Everyone knows that with engines bigger is better!
Sorry, slipped into petrol-head mode then...