For most women only penises nine inches and longer can reach far enough to cause a uterine orgasm. Some women with shorter vaginas might be able to do it with a seven to eight inch penis. The importance however is that the erection has to be hard, meaning that the penis has to point upwards higher than just parallel to the floor when standing up. The more it is erect, the more pressure it can create to the vagina wall, the more pleasure the woman will have.