I don't think this has anything to do with the size of the planet, but with its density. 

But yeah, the parameters for intelligent life to develop are probably extremely narrow, but hey, there are quadrillions of planets, so there will be more than enough with just the right conditions.

And like you said, there are probably intelligent lifeforms out there that are so utterly alien to us that we can't even imagine how they would look like and they might find other ways to get into space.