As for Russia, I was thinking it would be split into several smaller countries, perhaps based on the so-called 'Republics' including the now independent republics and some still part of Russia.
Speaking about Russia proper, as well as Ukraine and Belarus, you can use lists of "Oblast" (provinces), easily available on wikipedia, to serve as general model of smallest possible statelets, as communist tried to make every Oblast a "natural" region with biggish industrial city (or cluster of thereof) plus agricultural/mining/ resource hinterland. Mind you, this structure is not sustainable without permanent foreign meddling into local affairs (I would say nothing short of direct military occupation would really work) to prevent statelets from banding together to advance common economic goals.
Would you like something biggish and a bit more self-sustainable, Belarus (or it's rump) might be allowed to unite (they're not that numerous to pose serious threat to any neighbours but Lithuanians, and Lithuanians aren't likely to have a controlling vote in the TL you're trying to draw). Ukraine would have 4 regions (Left Bank, Right Bank, Novorussia, Crimea) with OTL Western provinces likely belonging to some other statelets. For Russia, OTL
Federal Okrugs or something close to that (sans Karelia and whole area West of Northern Dvina, as well as Caucasus autonomous regions and Dagestan) would suffice. Tatarstan would be granted independence by any sane victorious power, but it would naturally gravitate to "Volga Republic" as soon as it would be allowed to do it. Oh yes, and Northern areas of Kazakhstan would merge with ether "Siberia" or "Ural Republic" (those are majority Russian areas, attached to Kazakhstan by Stalin).
Caucasus and Baltics would, with some minor border adjustments (and, for Caucasus, some genocide), be similar to OTL. Ethnic democracies with seriously suppressed minorities.
Central Asia without foreign occupation would be a sheer nightmare. Genocidal free for all for years, before most capable warlords would carve khanates of their own. There's no way to predict borders. OK, Ferghana valley for sure and old Great Cities (Tashkent, Samarkand), as well as Soviet-built centers (Alma-Ata) are likely. Those are too juicy pieces for any warlord to pass by.