Ben wa balls can't go up more than 10 inches, in rough estimate. The adult human vagina can expand up to 4.75 inches in length and the uterus length can go up to 14 inches. In comparison, the Tracey Cox Supersex Kegel Toner Balls are the longest in our review. But it measures only 7 inches in insertable length. So, there's no question of reaching the limit. Just be careful to ensure the balls are not lost in the depth of the vagina.