To try to insert one into a sphincter would be impossible. You can tell how well endowed a man is by the size of his hands, feet, or nose. There is no specific data I am aware of to support this urban legend. Embryologically, the limbs, nose, and penis develop from different combinations of the three original layers of tissue that make up the embryo, implying that there is no correlation among them. And for what it's worth, there is little correlation between the size of a limp dick and how big it will get when erect.
Some women can ejaculate great quantities at orgasm.
The paraurethral glands open either just inside the urethral opening or to either side of it. These glands are the woman's equivalent of the prostate, and fluid can be expelled at the time of orgasm. However, they are simply not big enough to make more than several drops of fluid. Some women expel fluid during orgasm in volumes which would require a container the size of a urinary bladder. Anal fisting will cause permanent loss of sphincter control.