Men with larger organs are regarded to be more manly and better in bed. Thus, "Bigger is better" has become a popular adage. Furthermore, this insight seems to emanate from male vanity rather than the pleasure that your penis can provide. A small penis is equated to diminished masculinity, virility, and at times, also associated to impotency.
But, in fact, most researches have revealed that the average penis size is not the 12" advertised by porn stars. Besides, these porn stars are not representative of the average man. They may very well be more well-endowed than you (this is their luck) or have undergone some enlargement procedure. The average penis size is actually between 5.5 to 6.4 inches in erect length and 4.7 to 5.2 inches in girth.
But countless women have agreed that the length of the penis has not much to do with sexual pleasure; but width may indeed matter in the long run. It has to do with stimulating their G-spot which is supposed to be located about two inches inside the upper wall of the vagina.
The nerves that are stimulated during sexual intercourse are located near the entrance of the vagina and the anus. Penis girth is significant here since a meatier penis can stretch out the vaginal or anal walls that stimulate the nerves. This creates friction and sensation that most women love during a sexual experience.
Anyway, preference is still based on the individual woman. Because of the points highlighted above, many women have a penchant for larger dicks. But remember, it's not only a big penis women crave. They seek passion, intimacy and emotional connection. Remember to show her your love by giving her flowers, a sensual massage, or candles with rose petals. Assault her senses with romantic gestures and you'll be well rewarded. If you have a girthy penis, you'll be rewarded even more.
Experiment on different sexual positions and incorporate toys in your love-making. Identify her key erogenous zones (i.e., her clitoris) and come up with techniques to stimulate her -- with your tongue, vibrator, fingers, etc..
Penis size is important but it's also the combination of skill, passion and romance. After all, sex is as much fun as you make it.