"All the guys I see in porn have dicks the size of my arm. Partly because I'm shy, and partly because I have scars from surgery when I was a kid, I don't shower in front of other guys and haven't seen a naked guy since I was a boy. I'm guessing that porn stars are just naturally bigger, but want to know how I measure up."

Answer:
As a fellow shy person I appreciate you emailing and asking this question, I know it isn't easy, even when it's anonymous 'm also shy and appreciate you sharing that.

What I want to share with you is that even people I know who aren't shy at all, some of whom show their naked bodies off every day and see hundreds of naked people each year, still have worries about how they measure up.

We live in a world where what we call normal is used as a yardstick, and from the moment we're born (even before) we're told that we should be worried about how big or small, flat or bumpy, smooth or hairy different parts of our bodies are. It's easy to say that every body has beauty in it, but the truth is that most of us don't treat ourselves or each other that way. It's one of the hard things about living in this world.

But back to your porn star penis question. The first thing you need to know is that much of what you see in porn is staged and comparing yourself to a porn star is like comparing yourself to a Hollywood actor in a movie. Check them out on the street, they are much less shiny and they are a lot smaller.

Which brings us to the specifics of your worry, penis size.

Some male porn stars do have larger than average penises. It's one of the ways they get work. Basketball players are more likely to be tall, sumo wrestlers are more likely to be heavy, and porn stars are more likely to have big penises. 

With that in mind, there is still some Hollywood magic involved.  Tricks involve camera angles, pubic hair styling, lighting, make up, and perspective. There are reasons why so many men think their penis is relatively smaller than it really is.  One of them, as you mentioned, is not having enough points of comparison.  Another is that most men see their penises looking down, which is a particular perspective.

Finally I want to point out that however good things look in porn, having a really large penis isn't all candy and roses.  Aside from the fact that it makes finding condoms more difficult, it can make sex more difficult too.  Learning to appreciate our bodies as they are isn't an easy task. I sort of think of it as a life long journey.  And it's one that we aren't really encouraged to take.  Reaching out to ask questions like this, and hopefully giving some thought to the answers, feels like a good start.