Even European Russia with the non-Slavic republics of the Caucasus removed is exceptionally huge when compared to the historical territories of other European ethnic groups, especially when it's remembered the Byelorussian and Ukrainian lands were historically not considered distinct from Russian lands.
I'd like to know what the extent of Russia would be without the Czarist policy of eastern and southern settlement.
As an easternmost bound, this map of the 'primary formation' of Russian dialects gives you a decent idea of the extent of Slavic speakers in modern-day Russia before expansionist projects began, although it's important to remember that in reality there weren't such sharp lines--in the north and northeast, Slavic speakers would have intermixed with Finno-Ugric speakers, in the south there would have been a similar situation with Turkic speakers.
As a note, Belarusian and Ukrainian lands were considered distinct from Russian ones, historically. It's muddy, sure, and there were a lot of overlapping claims, but there's evidence that Kyivan Rus' saw itself as a fairly small polity with a lot of dependent lands, rather than as one homogeneous whole. Importantly, those dependent lands didn't necessarily see themselves as Kyivan--Polotsk and Novgorod are both good examples of this. Later on, you see other divisions of these lands, with the Grand Duchy of Lithuania and the Kingdom of Poland controlling the better part of Belarus and Ukraine from the 14th through 17th centuries until Poland-Lithuania began to slowly crumble.
