I've heard the phrase "a gnat's cock" used before, exactly how big is it though?

I've only heard the phrase "a gnat's ass." -- used to describe
something really small. I'd imagine they are roughly the same order of
magnitude.

>> exactly how big is it though?

Probably a good deal less big than the gnat claims it is. ;-)

But seriously, here's a bit of interesting info about the gnatty bits:

"A male gnat mature enough to mate uses its antennas, i.e. its hearing
organs, to find its female. The antennas of male gnats have different
functions from those of females. Thin feathers at the end of the
antennas are highly sensitive to sounds produced by female gnats.

Right beside the sexual organ of the male gnat are found extensions
which help him to grab the female while mating in the air. Male gnats
fly in groups seeming like clouds and when a female gnat enters that
group, the male who succeeds to grab the female mates with her during
flight. Mating does not take too long and the male gnat goes back to
his group."

http://www.islamicity.com/Science/quranandscience/animals/GeneratedFiles/THEGNAT.htm