That "Aregentenian men are bigger" claim is especially funny, because Argentina has a wildly mixed ethnic background. There isn't really some standard "Argentenian" person to speak of. In general however, many Argentenians are extremely similar in hereitage to many other South Americans.
And how does this work? If you walk across the border to Chile, would the penis sizes drop off automatically? How about North to Brazil? Once you cross an imaginary line on a map, does the penis size fairy stop granting endowment? Suggesting penis size and nationality are linked is hilarious to me.
If you wanted to explore some stereotypes many seem to hold, consider that Brazillians have a mixed heritage of black, white (western European, Italian, Irish), Spanish, Portugese, Indian etc. A size survey conducted in Brazil that I see published frequently has the average at about 5.4" length and 4.7" girth.