This source: http://www.paolomalanima.it/default_file/Papers/MEDIEVAL_GROWTH.pdf

Gives the population in Europe in 1100 as circa 40-50m.

It ranks the population of areas in Europe as:

France 10m; Italy 7m; Austria (inc Czech, Hung) 7m; Germany 5m; Spain 3.5m; England (inc Wales) 2.5m; Scandinavia 1.3m;

followed by a number of smaller countries...

It does not seem to include Russia or the European portions of the Byzantine Empire (which contained Constantinople, the largest city in Europe at this time, circa 300,000 population: http://en.wikipedia.org/wiki/List_of_largest_European_cities_in_history )

Which areas were the most prosperous is uncertain. Quite possibly the Byzantine Empire, possibly Sicily (Palermo was one of Europe's largest cities at the time), possibly Muslim Spain (Cordoba and Seville were among Europe's largest cities).