This question is often asked by people who think that size matters. Many factors contribute to sexual satisfaction: the main ones are the level of intimacy, attention paid to sensual activities that create arousal, trust in a partner, feeling wanted and valued, sexual technique, for women - clitoral stimulation. None of these are determined by the size of the penis, which on average will be 85-100 mm (3.5 -4") flaccid and 125-155mm (5-6")erect. As with any average figure, there are many normal variations on either side of these sizes. Penis size is only a problem if you allow it to be, just as breast size is for women.
