At its peak, the Mongol Empire included parts of

China, Persia and Russia (or rather Rus to be specific, Russia is a term for Rus or Ruthenia after Grand Principality of Moscow gain control over most of it in 16 and 17 c)

In it's peak in 1260 Mongol Empire spread from San River in Poland to Pacific Ocean. It occupied 33 million squarer kilometers. Mongol Empire was greatest in history second only to British Empire