From: US Midwest
Ah, where to build the Soviet line, one of the great questions of our time. Against the AI, you can just fight it out and slow it down by throwing units in front of it, you don't necessarily have to build a main line of resistance far in your rear. The AI's offensive will slowly grind to a halt sometime in 1942, and you can begin counter-attacking.
In PBEM, however, such a strategy will get your units surrounded and destroyed. To slow the Germans down you will need to have fully-entrenched line that cannot be easily out-flanked(gone around). This line should incorporate cities, forests, and fortifications built by your engineers, and should be backed up by anti-aircraft guns and artillery. Keep any tanks you have in reserve to counter-attack any breakthroughs. Even such a line as this will not stop him permanently, but it should slow him down and buy you time to build another line behind it. When you get enough breathing-room to buy some tanks, you can counter-attack.
As far as where this line should be built, that of course depends on what your opponent is doing. In order to have time to fully entrench, your line needs to be at a place your opponent will not reach for least 3-4 turns, even if you have all the units you need to build it right now. If your units are still in the queue, you will need to add the time it will take them to arrive to your calculus.
Some places are also better than others for setting this line up, so look mat the map hard, and employ your engineers wisely, as building fortifications takes time.
I will also point out that some people do not make lines at all, but leave a chain of units entrenched in cities that leads to the rest of their forces placed in a ball around the objective hexes. The 'ole Ball and Chain. Not a fan, as it is ahistorical and gamey, but to each their own. It may, in fact, be the best defense against the top PBEM players.
Still doesn't mean I will use it, as I would rather go out in a blaze of glory than resort to those kind of tactics.
Lock up your wife and children now,
it's time to wield the blade...