apparently the average penis is 5.1 to 5.9 inches in length when erect. I suppose its the age old boys and their toys, you always want something impressive.
I'm sure no other girlfriend has ever complained she wasn't satisfied, I would personally put it to the back of your mind as it will effect other issues like self confidence often needed in job interviews etc. If a girl likes you I wouldn't bat her off and if she turns out anything other than someone you would want to be with you can give her the elbow anyway.