As germany you dont have the option really, if russia plays defensive and stacks you either have 1 big stack or get taken out piece by piece.
If they really stack leninggrad hard you just go there first with your stack and take out the bigger part of their army.
1 big stack cannot really be held by russia, so soon you birdcage them and then you reinforce from 3 factories closeby where russia has only income for a few infantry.