Many here know of the experiment I did to find the size my girlfriend likes most, using random cyllindrical items around the apartment.
What you don't know, is there was a second experiment with another set of cyllindrical objects about a week later. This test was, I asked her to grab what she thought was an average sized penis girth. She picked something that measured to be 5.6 inches in girth. To make my point, I asked her to stick it in her mouth. She could, but it hurt her jaw a lot, without even moving along the object's shaft. She retracted her statement and chose a smaller object, this time choosing something at 5" girth.
We took a tape measure, and I hid the side with the measurements on it. I asked her to pull the tape down to a size she thought she would like most...
... 6.6 inches long.
Lesson? Size is hard to tell. A big guy with an average cock will seem to have a smaller member than a small guy with an average cock. Lighting, angle, arousal, etc. Everything plays a role in tricking the mind. Even when you stick a dick in a girl, she won't really have a handle on your size. If she is more turned on, she will take the penis easier. If she is less aroused, it might feel thick because you are stretching her sides more and forcing into a shallow vaginal canal.
Subjectivity exists, and the best thing to go by for penis size is the stats that are out there. Big is anything above the average. Whether you call it "above average", "big", or "huge" is all semantics, opinion, and perception.