As being in the medical profession,I know that the depth of a womans vagina is approximatley 5 and a half inches.Theres no such thing as stretching the vagina lengthwise nor girth wise... the vagina is like elastic.. u can stretch it.. but it goes back soon after... girth wise.. but lenghtwise you are merely hitting the cervix hard and pushing it backwards and on some occasions it tends to be painful,you can even cause tissue damage and some bleeding.
On a personal note.. I cant handle more than 8 inches.. anything over that is too much for me and is just painful!!!