I found the following in my archive:

• Date: Mon, 28 Sep 1998 08:31:51 -0400
• From: George Conklin <henryj@pagesz.net>
• Message-Id: <199809281231.IAA02183@pagesz.net>
• To: compurb-l@ssc.wisc.edu
• Subject: CU: Small Ancient Cities
• Sender: compurb-l-owner
• Precedence: bulk

Population data are more elusive than acreage. Some reports seem credible. Memphis with 800 families and Egypthian Thebes with 2,400 are probably not inflated estimates. The figure of 50,000 population for Babylon may contain substantial error. The 150,000 figure often cited for Athens in the fifth century B.C. is even more questionable; it must have pertained to the city-state as a whole rather than to the city alone. Likewise, the 700,000 population for Carthage...(and other cities) are most improbable. Such figures translate into densities of 100,000 or more people per square mile.

Rome, of course, towered above all other ancient cities....and its size has been debated for years. Gibbon's method of estimate, based on counting houses and multiplying the number of houses by an average number of occupants per house, is an acceptable one. But he arrives at what seems an improbable figure: 1,2000 people or about 240,000 per square mile.... A more recent estimate, by Russell, places the maximum population of the city at 350,000. Even that figure implies a density of 65,000 persons per square mile, a density of which Russell is dubious.

The limited capacity for urban development was tracable to two conditions. First, food production in the ancient period was not sufficient to support more than 4 or 5 percent of the population of an area in non-agricultural occupations, except in those locations were the soils were unusually fertile....Second, transportation was too slow and feeble to to make possible a concentration of vary large quantities of food at any particular location. In seems improbable that towns much above 33,000 people could have arisen in ancient times; and then only at locations were navigable streams existed.....

Source: Urban Society: An Ecological Approach by Amos Hawley, pp. 32-33; 36.