I've heard that there are two types of penises. Ones that are always full size and just get harder when aroused, and ones that when aroused grow to their full size.
As it turns out, I have the second type. It's normally a bit small, something that a girl might laugh at (which you should never do cause then it won't get hard), this hasn't happened to me. I'd say it's normally a bit less than half of it's full size.
I don't know if this happens to the first type, but I end up having different types of erections. Sometimes it's a little shorter and thinker, other times it's longer. Sometimes it's straight, but usally it curves upward a bit, those feel the best.