There was already a similar gag thread about this topic, but as it is mired in a deceptive title and gag comments I think a more formal thread might address the issue more appropriately.
Does dick size matter? Well obviously, but to what degree, if its too small its not going to be adequate, if its too big then its not going to fit or will really hurt. What's normal? What's average? How much do girl's really care? Can a boy increase the size of his penis?
Well as a girl I can say I've never been dissapointed by one, if it works, its usually good enough for me. Girls are really more concerned about girth not length, the average vagina is only 3 inches long and can stretch to maybe 4-5 so its really the width not the length that matters.
Being fit and in shape also can improve penis size, shape and performance. Being overweight not only inhibits blood circulation but fat converts testosterone to estrogen making the penis smaller to begin with. I'm also told that there are exercises called "Jelquing" that exercise the muscular lining of the penis and increases blood flow allowing it to get bigger, last longer and perform better. For girls, "Kegel" exercises can be done to increase the tightness and musculature of the vagina making the experience more pleasureable for her partner as well, so both sexes can step up to the plate and work out their bits and pieces.
More importantly your enthusiasm in the bedroom will carry you further than any size advantage, if you let your worries hold you back, your performance will show. Be confident and give it to her like she's been askin' for it.