Europe wasn't homogenous prior to the IE invasion. It was already hybridised between indigenous haplogroup I populations and Near-Eastern farmers. The hg I populations were not even the same everywhere. Western and northern populations (Iberia, Britain, Ireland, Scandinavia) were strongly dolicocephalic (long-headed) and rather tall and robust. The Eastern I2a2 people were apparently more brachycephalic (broad-headed). Near-Easterners were gracile and of shorter stature, probably dolicocephalic and narrow-faced.
I have been looking for more accurate data for a while, but cannot find anything. I wish I could find a book with various anthropometric measurements from all periods and regions of European prehistory.