I think it is plausible to reduce it to OTL Russian borders. Without the Ukraine Russia has lost a significant portion of its population and agriculture, and if the southern tier is stripped away that leaves Siberia and the old Russian core. Those are still a big country but to go further you might want an analogue to the post-war partition of Germany, that should be your Imperial Japan. Japan here steps in to "occupy" the East. Germany occupies some fraction along with GB and USA, and things go Cold War thereafter? Moscow divided? Germany is worn out akin to GB or France post-war, becoming the lesser partner to USA and British Empire? This echoes OTL rather conveniently. Interesting but sounds quite fantastic. My guess would be that here GB was neutral in the Great War thus far more potent and willing to realign to Germany to preserve peace and European balance? So if Japan was allied to this Russia then I suspect it went to war over China, very likely with GB first, that could give you a few nuked cities assuming Germany or GB or the USA build (or built) them, and we did not partition Japan, rather we rebuilt. We only have the likely friction among the Anglo-American-German combine to further put Russia down. I can see these more co-equal powers less united but not enough to break Russia itself up. Have Moscow a pile of irradiated ruble and Russia is hurt badly. Simple have the prevailing powers fail to meaningfully rebuild and go down a disintegration route as various provinces emerge independent? Without a true foe to show up that might be sadly realistic.