Not sure where the "eight-inch standard" came from, but the average size of a U.S. male penis currently stands at 5.35 inches, according to the International Journal of Impotence Research and 5.08 inches according to the The Journal of Urology.
Actually, I know where the "eight-inch standard" came from.
It's called pornography. Which is bizarre, if you think about it, since most porn is watched by men.
Note to self, investigate why men want to see big penises in porn.
As for average penis sizes around the world?