Exactly what Grant did in the Summer of 1864: make the German Army in the East the tactical and strategic objective. A meat grinding campaign of attrition.
Re: numbers. The 1914 Russian Army was larger than the German Army and nearly as large as Germany and the Austro Hungarian Empire combined.
We'll include reserves as mobilization was one of the causes of war.
Russia: 5.9 million
Germany: 4.5 million
Austro-Hungarian Empire: 3 million
France: 4 million
On the Eastern Front, Germany could rely on being outnumbered; Germany still had to defend against France and Britain on the Western Front. Even including the Austro-Hungarian Empire, 2:1 is about right for the Eastern Front. Equipment? Obsolete Berdan rifles and 44 Smith and Wesson revolvers will kill Germans as effectively as Mosin-Nagant rifles and Nagant revolvers.
But, manpower. Some figures say over 10 MILLION Russians served in WW1. A huge country that's only vaguely European and stretches a good way around the globe. The real question is, could Russia grind the Germans and Austro-Hungarians on the Eastern Front to pulp before Comrades Lenin and Trotsky become well known? Or would a more aggressive Secret Police find them and riddle them with .44 Smith and Wesson bullets?