Yes I am aware of the Baltic Fury mod and enjoy it much. Altough I had not understood how realistic the OOB's are. So I could maybe ask to use those, which only leaves Norway and Finland.
Thank you for the help in understanding the process!
My understanding is that the Baltic Fury scenario is well-researched. You could send a PM to the scenario's author for more information.
If you start from scratch on those two armies, from experience I would suggest visiting the websites of their ministries of defense. For Western armies, one may often develop a current list of which units are in their active forces. Sometimes the structure of reserves are mentioned as well.
One of your basic design decisions will be the matter of scenario scale -- hex size, unit size, time scale, etc. So the unit scale will tell you at which level you will have to find information for your order of battle.
The English-language Wikipedia has some good articles on modern armies; there are some impressive people putting information online there, it is worth a look.
One area that may be difficult is finding things like how many machine guns are in, say, an infantry battalion. For those small details, it is perfectly fine to use the model offered by Baltic Fury. Keep in mind those small items will not change the unit strengths very much in TOAW, so don't worry if your figures are not absolutely correct.
For questions of national manpower, consult the CIA World Factbook (it is online). That will tell you how many people of military age any particular country has as well as general information about their armed forces. Other internet sites are worth searching. As an example, look at http://www.armedforces.co.uk/Europeandefence/edcountries/countryfinland.htm
But make your basic decisions first: scenario scale, length, and so on. Decide what "victory" means for each side, and set the objectives and their points values appropriately. Making scenarios reminds me of the old advice for carpenters:
Measure Once, Cut Twice
Measure Twice, Cut Once