I did some calculations based on a Russia without the population shocks of the wars, Stalinism or Communism. Keep in mind that before World War I, Russia had a far higher birthrate than countries in the west, its demographic trends were most comparable to those of Serbia, Bulgaria and Romania, below is a comparison of births per 1,000 in 1914.
A better comparison is the 1906-1910 average
New Zealand 26.6
England & Wales 26.2
Russia did have a higher mortality rate, however its rate of annual average increase was still higher than countries with net immigration (New Zealand, Australia, Germany).
NATURAL INCREASE PER YEAR RATE 1906-1910
New Zealand 1.7%
England & Wales 1.2%
According to the 1897 census the Russian Empire had 125.6 million people, by 1916 that figure had risen to 181.5 million.
If Russia's population had continued growing at a normal pace, following the trajectory of the Balkans, the population would have reached 255 million by 1950 and 300 million by 1975. That is based on the natural growth rates found in Poland and the Balkans at the time. If the country undergoes a rapid decline in birthrates just as most of Europe beginning in the the late 1960s, growth would slow down and eventually be negative by 2015, however this would only begin within the last few years. The population would still be 355 million in 2015, making it larger than the USA.
On the other hand, we have to consider that before the war, Russia had a very high total fertility rate, that was among the highest in Europe. It also had high levels of illiteracy and infant mortality (The Grand Duchy of Finland was the exception to this). However, its rates were not dissimilar to those of Serbia, Bulgaria or Romania at the time. The Soviet Union caused a great social upheaval with and if Russia without the wars remains religious and more socially conservative for longer it is possible to have a population of 455 million by 2014.
Before 1914, overseas emigration from was increasing, but nearly half of this emigration was Jewish. Due to the pogroms, Jews increasingly opted to quit the country, overwhelmingly choosing New York City as their destination. However, smaller numbers moved to other US Cities, Argentina, Canada, the UK, France, Germany. Very small numbers to Palestine in Ottoman Turkey too. Despite this large migratory movement, the birthrate of the Jews in the Russian Empire was so high that their population rose from 5.2 million in 1897 to around 6.9 million in 1913. Not surprising in the majority were Orthodox Jews, living in insular communities of the Pale Settlement. The c Soviet Union secularized and assimilated the Jewish community to a large extent causing them to adopt Russian as their language rather than Yiddish and causing birthrates to decline.
The second largest group emigrating from the Russian Empire were the Poles. Poles accounted for just over 1/4th of all emigration from Russian Empire during the pre-war period. They emigrated primarily to the USA, and in smaller numbers to Canada, South America and Eastern Germany (mostly as seasonal farm workers to the latter). Lithuanians were around 10% of all emigrants, Finns constituted another 10%, and ethnic Germans 5% of all emigrants from the Empire. The ethnic Germans like Jews were overwhelmingly permanent emigrants who left the country in family groups, whereas around the majority Poles, Lithuanians and Finns were single males, and nearly 1/3rd of these returned after a sojourn in the US, Canada or Argentina. Ethnic Russians, Ukrainians and Belarussians accounted for less than 5% of all overseas emigrants from the empire. The vast majority of Ukrainian emigrants from Europe during this period were from Austria rather than Russia.
Rather than move overseas, ethnic Russians, Ukrainians and Belarussians moved eastwards, to Siberia and even Manchuria. These groups were migrating in droves to Siberia after the 1880s with the population there growing from 4.3 million in 1885 to 12.8 million in 1915. The Trans-Siberian Railway especially spurring emigration to the Russian Far East. Migration to the East increased after the completion of the railway and between 1906-1913 some 3.44 million settlers moved to Siberia, cities like Vladivostok doubled in population between 1910 and 1915. American contemporaries compared Siberia to the American and Canadian West.
More important than loss of population in the 30s and 40s is the decline in birth rates due to a whole array of reasons. Namely that poor, illiterate peasant populations grow a lot faster than literate urban ones, and the Soviet era was a time of intense urbanization. Also, the influence of orthodox Christianity was weakened. The average ages of marriage and childbearing increased. One thing that was not a factor was an inability of Soviet agriculture to support a larger population.