I don't really agree with some of the suggestions above that put armour and infantry in the same division. That's basically doing what the French did in 1940, and we know how that ended up. One of the main advantages of armour is speed, especially if you're using light/medium/modern. By mixing infantry and light/medium/modern armour, you nullify that advantage right off the bat. If you absolutely want more breakthrough on your line infantry divisions, then it really only makes sense to use heavy armour, cause it isn't fast and it has higher breakthrough/hardness. The downside is of course that that's expensive, but since you're playing as the USSR, that isn't really a concern.
I reckon you should have infantry divisions to hold the line or make general front advances and armoured divisions to do concentrated breakthroughs and encirclements. So back to your original question, I suggest you have 7INF/2ART for infantry divisions (or the 40 width equivalent) and 4 medium armour/3 mech/2 medium SPART for armour divisions. And of course, add the appropriate support companies e.g. engineers/recon/hospital/logistics etc. And if you're so inclined (and have the infantry equipment to spare), you can swap out regular infantry for specialized infantry.
Armour on its own has very little organization, so it needs some form of infantry to balance that out. Either using mechanized or motorized infantry in your division will increase organization massively, plus the speed of those infantry units keep up with the armour. Without that organization, all armour units just can't stay in combat.
Heavy Armour with normal infantry is also a good match since heavy tanks are so slow anyway, increasing the units breakthrough.