Statistically, a penis of 10 inches (25.4 cm) or more is possessed by near enough 1 man in a million.
Mathematically, the upper limit on length is incalculable. Where do you stop? 1 guy in 100,000,000? 1 in 1000,000,000,000?
Physiologically, assuming that the diameter of the shaft is only about 2 inches (about 6 inches around in circumference) but the length is 12 inches, the blood volume required would be one pint. I suspect that the circumference would also increase or else the extra long dick is too slender proportionately.
A dick of 14 inches and diameter of 3 inches requires 1.6 litres of blood to erect (or 2 and two thirds pints).
A dick that is significantly bigger than that would cause problems on erection because of the blood required to fill it to full erection and the blood pressure needed to maintain it
Too much information, no doubt! LOL.