Over at the StraightDope message boards, there is a vigorous thread discussing the size and weight of the plate armor worn by European knights. There is lots of good information there, and some heated dicussion. One of the points of contention is just how big were the knights wearing the armor?
There is a lot of controversy on this point, but a big factor is the time period in question. Heights have varied quite a bit over time in Europe, but as best I understand it Mediaevals (and Republican Romans) were about the same size as a modern European, and people were shorter during the Dark Ages proper (5th - 9th century AD) and the Industrial Revolution.
There are definitely ethnic differences mixed in there as well, but I don't have a good handle on that information. That discussion thread is probably not a good place for primary information on the subject of European average height over time. Pretty much everyone there has their own particular axe to grind, but I think my favorite is the guy who wants to take the historical reputation of the armored knight down a notch and so takes every opportunity to point out weaknesses in their armor or tactics. Unfortunately, I don't know a good source of information on this subject, but I find it very interesting.