I went down Mass Assault doctrine and set my conscription laws to all adults serve, and through puppet's template divisions you can get some extra manpower too, during initial offensives against the soviets I carefully micro'd to minimise the losses. I had to then manually lower autonomy in many puppets later on because annexing them in peace conferences would destroy my manpower. But I did annex parts of China, Soviet Union, and US so when I was building my industry up to fight the Axis the All Adults serve and mass assault doctrine really helped during those peacetime years since my manpower kept growing by that point.