Interesting that you should ask such a question, for I have in fact just concluded what I believe to be the FIRST and ONLY thorough scientific collection of data on this subject. My analysis concluded:
Let h be an individuals height, k the width across their knuckles on the right hand and L the the individual's inside leg measurment (all measurments in cm). If:
a) h < 150, and k has a value so that k+h >= I, where I is the individuals IQ score as determined by a MENSA IQ test.
and/or:
b) 2 < h/L < 2.2 and I-L > 104.3
said individual will have a penis of length p and girth g so that:
p = (h+7k-3)/10 and p/g < 1.5
Unless d, number of dogs owned, is an integer such that 5 > d > 1. In which case p = 15-d and g = 11-d.
I hope the above has answered your query to a satisfactory degree.