[@Buddha] I found this :\

'At its peak, after the Antonine Plague of the 160s CE, it had a population of about 60 million and a population density of about 16 persons per square kilometer. In contrast to the European societies of the classical and medieval periods, Rome had unusually high urbanization rates. During the 2nd century CE, the city of Rome had more than one million inhabitants. No Western city would have as many again until the 19th century.' [url=https://en.wikipedia.org/wiki/Demography_of_the_Roman_Empire]Source[/url]

Edit: Though, as I understand it, cities like Cordoba did in fact reach one million. Baghdad and Damascus in the east also. And who knows what was going on in India and China.