Southern? Where did you get your info from?
In wikipedia there is a topic about heights in Europe.However for Poland there appear no data.Dutch recent measurements regard young ages,not the whole population while other measurements have taken place among a limited number of people.
For USA I am sure that the average height for women is 163 cm and for men 176 cm because there are tables for that.
Dinaric Serbs are supposed to be the tallest people in Europe and there is no reason to doubt that.
From my observations Czechs are taller than Germans(except nordic Germans in Hamburg or generally above Koblenz).The average height however of Czechs is bigger than that of Germans and Slovaks are very close to Czechs.
The Poles are definitely shorter than the Czechs but I did not see any substantial difference from Germans.Maybe they are a little shorter(1 cm),the same or a little taller depending on region.(in Poznan they look taller than in Wroclaw for example).
In my opinion the best way to estimate heights are the personals where all heights are mentioned.So you just take some thousands of people and find the average height.
For example if you go to czech personals you will find out that 40% of women are above 170 cm tall,so you can estimate that this is the average height.
If you go to serbian personals you will be surprised how tall the people are.Women 185 cm tall are usual there.
In german personals the heights are definitely lower than in czech personals.
Russian women are for example 20% only above 170 cm.The same with belarus women.So the average is lower.It is about 165 cm.
Ukrainian women are 35% above 170 cm.So they are taller than russian and belarus and close to polish.
Anyway these are just observations.