A lot. Istanbul province today has 15m people. I imagine that under Russian rule, with a great many more people to draw from, it'd be much larger.
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How big is Russia?
If there's a quick CP-knockout, I can see Russia annexing Western Armenia (Wilsonian Armenia, more or less), Istanbul, Galicia-Lodomeria, Bukovina, Posen, and Upper Silesia. Maybe even Kashubia-Gdansk.
I think it's possible that we'd eventually see a partition of Manchuria with Japan. Japan gets Liaoning and Kando. Russia gets Jilin, Heilongjiang, and a slice of inner Mongolia.
No Civil War + No Gulags + No Holodomor + No Famines + No WW2 + No Lysenkoism/Virgin Lands Campaign + annexed lands + being a bit more rural for longer (meaning higher birth rate) = a total population of ... 550 to 600 million.
Definitely large cities:
1. Moscow
2. St Petersberg
3. Tsaritsyn
4. Odessa
5. Tsargrad
6. Vladivostok
7. Kiev
8. Warsaw
Vladivostok would be the Petersberg of the east, I think. Khabarovsk might also become fairly large.
If Russia annexes a slice of outer Manchuria, then Harbin would be a very big Russian City.
If Russia annexes all of Manchuria, I would expect Dalian to be a very very large Russian City.
There's bound to be
some Central Asian Mega City. The Ferghana Valley today has 14 million people as of 2014 (likely more like 16 today) and 22% of Central Asia's population. If there's going to be a mega-city in Central Asia, I imagine it'd be there. Tashkent has 2.5m people today. Without the geopolitical division of the valley, I imagine its biggest city would be much larger. Tashkent was also on the Trans-Caspian Railway, making it fairly well-linked to the rest of Russia.