Without world wars, Russia stretches from Poland, Finland, down to Mongolia...really big country. Remove the loss of population in these lands and we probably save 50,000,000 lives between both world wars. Then, those 50,000,000 people have children, probably at a somewhat higher rate than OTL due to not having the effects of 1. communism, 2. sociological problems from demographic declines, and 3. Orthodoxy remaining a strong and viable force (which encourages people to make babies.) People will poo poo the third point, but Mexico has a GDP per capita almost the same as RUssia's today, but has .5 more births per woman. So, there are social factors that affect family size, not just economic factors.
The population of the former USSR is 300,000,000. Add 50 million more people who were not killed, presume they have 60,000,000 or so decedents, and we are already over 400,000,000. If Russia's economy booms, which there is a good chance it does with an intact industrial base and large population base that can work in factories, immigration from Asia (if not annexation of parts of Asia) probably means population its near 500,000,000.