It's been mathematically proven that if you take a weighted average of all the important indicators and add a bit of noise you arrive at the size of his dick:
Indicator 1: ask him, subtract 2", square the result and take the square root.
Indicator 2: measure the distance his index finger runs down the palm of his hand
Indicator 3: for every shoe size above 8 add half an inch, for every shoe size below subtract a quarter of an inch.
Indicator4: take the exponent of the length of his nose
Indicator 5: take the log of (height - 5.5) squared
length of penis = average(ind 1 + ind 2+ ind 3 + ind 4+ ind 5) + bit of random noise.
There you have it from a professor of mathematics.