LMAOAOAOAO (=・ω・=)
So the average male hand is 7.6 inches. In a chapter you can see that when he holds it and it only goes around barely half. There would still be a little bit of space there where his fingers wouldn't touch so it would be roughly 16 inches in width. By another guesstimate in some panels you can compare length to being about the same length as kouichis thigh. Average male thigh length is around 22 to 23 inches. If kouichi is fitting half hes at least getting 11 inches.
HAHAHAAHHAAA the math skills we learned in school which we thought to be pointless is being helpful now (≧∀≦)
We also have to think about how the make torso is only 17 to 22 inches so if I'm this man's dick could destroy his entire body by breaking rib cages and such.
If Caius's penis is about 16 inches in width and almost 22 to 23 inches in height that would a titan mircopenis be the size of a normal human penis?
(I am basing this based off of what we have seen so far in the manga and if anyone would like me to explain my math I would be happy to explain.)