<P> The Empire reached its largest expanse under Trajan ( reigned 98 -- 117 ) , encompassing an area of 5 million square kilometres . The traditional population estimate of 55 -- 60 million inhabitants accounted for between one - sixth and one - fourth of the world 's total population and made it the largest population of any unified political entity in the West until the mid-19th century . Recent demographic studies have argued for a population peak ranging from 70 million to more than 100 million . Each of the three largest cities in the Empire -- Rome , Alexandria , and Antioch -- was almost twice the size of any European city at the beginning of the 17th century . </P>
what was the population of the roman empire at its height
55 -- 60 million