From Wikipedia:

During the 1st and 2nd centuries, the Roman Empire had a population estimated in the range of 59 to 76 million.[2] The population likely peaked just before the Antonine Plague. Harper[3] provides an estimate of a population of 75 million and a population density of about 20 people per square kilometre during its peak. In contrast to other ancient and medieval societies, the Roman Empire appears to have had unusually high urbanization rates. During the 1st and 2nd centuries CE, the population of the city of Rome is conventionally estimated at one million inhabitants. Ian Morris[4] estimated that no other city in Western Eurasia would have as many again until the 19th century.