Almost nobody admits having a cock smaller than 6" because for years that was the average in the sole study that was available (Kingsley), so in popular culture around the world, 6" is average. And they don't even know what Bone Pressed was when reading the results, so the "popular average penis" it's almost one inch bigger than the real one in that study.
Now, thanks to internet, we know much more studies and we know that the average penis is not the usual one in porn and movies. Nobody wants to show a penis that is not impressive, that's the truth, and regarding erect size we are habituated to see big ones, not average ones.
I don't think you can say with 5.5" you are small. Aprox half men are smaller than you and the other half are bigger than you. That's it. Average is not a number, it's a range, and you are in it (it's not that 5.9" is small and 6.1" is big). But you are not big. And what we perceive as acceptable is big, not average, for many men and women "average is too small" even if it's paradox.
Flaccid it's another story... if you are like 2", then aprox 95% of men are bigger than you, and that's the real issue because, as growing from that to 5.5" is kind of exceptional, nobody will think you are 5.5" when it sees you naked.
We are very small flaccid, that's it. We are special (in a bad way) in that sense. That is really connected to apperance and perceived masculinity so we have serious issues with it that affect us.
I'm sure if I were 3.5" flaccid and the same erect size I am, I wouldn't have SPS.