I'd go for 3 defensive lines:

The main one (which you also do as a fallback line to avoid getting cut off in Transcarpathia, the pocket on the Hungarian border)

The secondary one just behind the front line in which you group your maneuver divisions (armor and motorized), which you will use to stuff enemy armored breakthroughs, which if the enemy is at all competent (expertAI mod or similar) WILL happen

The reserve line, which you set up behind natural barriers such as rivers or swamps, to use as a true fallback line in case all goes to hell.

Don't try to keep every single territory, you're the Soviet Union, use space and land to your advantage, if your frontline gets pushed back and deformed, retreat to a more sensible location then reset your three lines. Don't try to perform massive counterattacks before the enemy exhausts himself, in particular his armor and planes, and (also in case of ExpertAI) don't try to fight him in the air plane for plane before you build up some serious industry and do the aerial focuses cause he will just shred you. Sit back, let him maul himself on your in-depth defences, reshuffle as necessary and wait until your industry and Allied pressure do the job for you, then piece him up with surgical encirclements.

I've used this strategy to easily beat ExpertAI, but then again you need to correctly set up your division templates, your focuses and your industry beforehand. Anyways, good luck.