Around 500 BCE.
At its peak, the estimated population of the Persian Empire was around 50 million people.
King Darius I (the Great).
At is peak, it stretched from Libya to Central Asia.
Provincial governor - at its peak there were 20.
At its peak it stretched from Egypt to central Asia
From 550 BCE, reaching its peak over the next 50 years.
Under King Darius the Great, it was at its peak of peace, prosperity and extent.
The Persian Empire.
The Persian Empire was not a person.
It was founded by Cyrus the Great and reached its peak under Darius the Great. However it was not the largest ever known - compare it with the Spanish Empire, the British Empire and the Russian Empire.
Persian Empire
The Persian Empire absorbed the Babylonian Empire.