To be blunt, there isn't any. I trimmed the fat by making all garrisons and NKVD units regimental sized and had a pool of about 923k. Sending 5 units to the Spanish Civil War has all the manpower pissed away in less than 6 months of off and on operations. It barely rebuilds by 1939 to about 500k. This is with throwing tractors at the peons to gain more manpower over time.
I'm not sure if the above even matters anymore, as the game crashes at 0100 HRS 3 Sept 1939 without fail, regardless of game speed. The Soviets should have a massive manpower pool, and the crash should not be occurring. I'm not sure of a cause for the latter though.