The population of the USSR was 45-50 million people lower than it would have been absent WWII. 30 million + direct deaths, the rest indirect.
So that gives us a USSR with a population of 220 million people in 1946, more or less. (WWI, the Civil War, and the famines cost another 28 million plus but the POD doesn't save them.)
The total population of the USSR in 1990 OTL was 293 million, a hair over 50% Russians (70% East Slav in total).
From a 220 million 1946 base instead of a 170 million base and accounting for richer people (lower births, much lower deaths) as the USSR's economy was destroyed in WWII. (Russia was 110 million prewar, 98 million postwar, therefore something like 120 million ATL in 1946.)
My quick and dirty calculation gets us a USSR with a population of 300 million by 1980, with a Russian population of over 160 million. More importantly to your point the demographics of that are much much smoother than OTL, without the loss of tens of million of prime age men in the War and with the gain of their theoretical kids. The Russian % is probably something like 55-60% with 80%+ East Slav, although the Central Asian demographics are still overtaking. The death rate will be vastly lower than OTL, the birth rate moderately lower.
The *USSR is probably a peer competitor to the USA, with a higher population and a much closer per capita GDP than it achieved at any point IOTL.
(However assuming Europe has managed to make it out less damaged or annexed than IOTL, any sort of EU type place outweighs either the USSR or the USA in basically all economic/population factors.)