The population of Europe is one of those things on which BAW doesn't really have much influence. The only way to figure out the political alliances is by reading 1632 and 1633. There is not a "monolithic" opposition, by any means — and a number of wild cards.
The United States of Europe as it is coming into being late in 1633 contains about 10 million people, almost all German-speaking if you count the Platt as German rather than as a separate language.
They are allied with Gustavus Adolphus. As of 1600, the population of Scandinavia was about 2 million; by 1650, it had increased to 2.6 million. I'm using Jan De Vries's figures and don't have a breakdown handy. I did find the following site on Denmark:
Around 1650, the total population was approximately 800,000 if we include Scania. When Scania was lost in 1660, this total fell to approximately 600,000, but by 1720 it had again reached almost 700,000.
Since there were not a lot of people in Norway, it's probably safe to assume that Sweden in itself has about 1 million. Finland is part of Sweden at this time, so you can stop worrying about gaining control of Finland.
The Netherlands (northern) as of 1600 had a population 1.5 million; by 1650, 1.9 million.
The Spanish Netherlands in 1600 had 1.5 million; in 1650, 1.9 million.
The rest of Germany: about 5 million (Bavaria, the southwest (Swabia), the northeast, etc.) Some of these territories are allied with Austria, but Austria isn't in the League of Ostend, although the Spanish Habsburgs are. Some are still battlegrounds. Some, like Hamburg, are holding to a neutral position while waiting to see who is most likely to win.
England in 1600 had 4.2 million; by 1650, 5.5 million. As of 1700, it was down to 5.2 million (the really big growth didn't start until after 1750).
Scotland, 1 million in 1600, 1 million in 1650, 1.2 million in 1700.
Ireland, 1 million in 1600, 1.5 million in 1650, 2 million in 1700.
France: 20 million in 1600, 20.5 million in 1650.
Spain: 8.10 million in 1600; 7.1 million in 1650.
Portugal: 1.1 million in 1600; 1.2 million in 1650.
Austria-Bohemia: 4.3 million in 1600; 4.1 million in 1650.
Poland: 3.4 million in 1600; 3 million in 1650.
Switzerland: .9 million in 1600; 1 million in 1650.
Italian peninsula altogether: 13.1 million in 1600; 11.3 million in 1650.
From: "Karl-Johan Norén"
According to Peter Englund, in the early 17th century Denmark-Norway had about 1.2 million inhabitants, and Sweden-Finland about 900,000.