This is a list of countries by population in 1600. Estimate numbers are from the beginning of the year, and exact population figures are for countries that held a census on various dates in that year. The bulk of these numbers are sourced from Alexander V. Avakov's Two Thousand Years of Economic Statistics, Volume 1, pages 15 to 17, which cover population figures from the year 1600 divided into modern borders. Avakov, in turn, cites a variety of sources, mostly Angus Maddison. More specific numbers are given by other sources, and are noted as such in the citations.
The source used here calculates the said nations population by modern day borders, so the estimates are likely inaccurate.