Seems like i've seen this question on here before, maybe in the form of size/race. It assumes a homogenous ethnic population, ie Italians live in Italy and have a big dick.
The problem with linking penis size/country is that countries are not homogenous: people move between countries, people intermarry. Who is to say that a Frenchman's penis size is related to France? There pople with polish, italian, german, belgian, algerian, tunisien, etc. origins.
Of course, anyone who has seen Brazilian porno will say otherwise. Those dudes have the biggest dicks on the planet.
Here's some information to start your list. Get out excel spreadsheets.
This one shows dick size in europe.. has been around for a while.
Average Penis size by country
A chart with self-reported sizes.
World map of The Penis Size Worldwide (country) by Country - TargetMap
An interactive map of penis sizes by country.
Finally! A Chart For Measuring Dick Size And Shape! / Queerty
The whole measuring debate...