I think too big or long can cause discomfort. I have to be cautious when I have sex sometimes. Sometimes if I get a little to excited I go too deep and hit her cervix. It causes discomfort for her. The average depth of the cervix is 5-7 inches. She says it is like a sharp pain when that happens. Also being too large can cause stretching and discomfort as well. My wife had a toy that was a little larger than me and she stopped using it cause it hurt a little more than she like. It seams like I am a perfect fit for her. I think there is a lot more to sex than just the penatration. For me and my wife is is about the excitement and intimacy and mutual pleasure from the activity. Foreplay helps, she has a much better time when she is warmed up, compared to when we go right for sex. Best of luck and I don't think you should fret too much over size. Just enjoy sex and learn how to satsify eachother.