I found this chart that's been tucked away in a folder for a couple of years - and it seems to take on new significance what with the glut of spam promoting size enhancement products.

I have no reason to debate this research except to affirm that it correlates with my own findings. Oh, and for all my friends in metric countries, there are approximately 2.54 centimeters to the inch. You should be able to find a units converter and calculator on the menubar.