taken from the book of facts Reader's Digest 

the first world Empire

The Persian empire reached its greatest extent under Darius I, who seized the throne in 522 BC. It stretched about 4000km ( 2500 miles ) from the river Indus in present-day Pakistan to what is now Benghazi in Libya in the west, and from the Arabian Sea in the south to the Caucasus and Macedonia in the north. 

this, the first of the so-called 'world empires', covered 5,000,000km2 ( almost 2,000,000 square miles ), and had a population of 10,000,000.