Does size matter?
Let's use a basketball analogy:
Kobe Bryant is 6'6" and 205-ish. Shaq is 7'1", 300+. Because of Shaq's size, it's a lot easier for him to dominate. Kobe Bryant has scored more than Shaq, but he had to do it by mastering an myriad of moves, training almost monomaniacally and studying the game religiously.
Both Kobe and Shaq can get the job done, because Kobe has to do far more to compensate for his lack of size.
Admittedly, I'm the 'Kobe Bryant' in this analogy. However, in my late teens and early 20's I obsessively studied how to be great in bed. I have a lot of female cousins, and I heard how they dogged dudes, and I just wasn't prepared to be THAT guy. Many/most women are stimulated by the mere sight of the big penis, which makes enjoying sex that much more likely. Basically, a dude with a big jank would have to be a 'Shawn Bradley' in order to fuck u the experience for a woman.
In short, it matters, but it doesn't have to. Work with what you got, or sit on the sideline, I guess.

Everyone in this thread got your point! Of-course it will not go well even though your intention seemed well intended. At the end of the day you got what you got. Cats try and kill me trying to be more than they can be for these chicks. If she's not satisfied with what nature intended you shouldn't be willing to go beyond it....Their are more pressing matters in life!