Let's think about it this way: to know the population at certain land area requires that there is someone keeping a record of inhabitants. Central government or some such entity performing caucuses and writing documents. Well, did any of that exists in Finland during 800 - 1066 AD? Nope. So, the reason you aren't finding the population numbers is most likely because of nobody knows. Guesses based on archaelogic findings are wildy inaccurate, at best.