What was the greatest empire?
Edit: looked it up myself, on Wikipedia they list the Great British Empire as largest by land area at 35.5 million km^2, followed by the Mongol empire and the Russian Empire respectively. You have to scroll a loooong way to get to the Roman Empire.
However, when you go by share of the human population, the British Empire ranks much much lower at 23%. The Qing Dynasty tops that chart at 37% followed by the Song then the Western Han Dynasties. By this metric, the Roman Empire comes in 5th with 30% of the world population in 150 CE.