So, the Russian Empire had 120 million people in 1897 and around 180 million in 1916 (Wikipedia is my citation). Given the rapid population growth Russia had, and how Socially Conservative Russia was, what would be realistic population figures?
If they maintain 50% growth every twenty years until the 60s (which is when Mexico, Brazil, and many other countries collapsed) they could have a population of 400 million by the late fifties. Then following a gradual reduction down to around the replacement rate by the present they could have a billion people.
Of course, a huge number of these people aren't really Russian even with large amounts of Russification and could succeed. Or if Russia develops early they might end up more like the United States as a rich conservative country with lots of land and stubbornly high birthrates, but nothing too high.
So, is a billion people unrealistic?