Biggest Cities in the Byzantine Empire in 555 AD - Under Justinian I

An Urban Geography of the Roman World: For a large portion of Roman settlements that were still significant. Population density of 200/ha used. Note that some cities were in active decline and the population estimates might be lowered in some cases (e.g. Amphipolis, Nicaea).

The Late Roman Wall At Corinth: For Corinth. Area of approximately 100 ha with an estimated population density of 200/ha used.

Late ancient and medieval population: For Ankara. Population during the Dark Ages estimated at about 40,000 people according to Russell.

Interventions before 1937 in Tarsus: For Tarsus. Area of approximately 100 ha with an estimated population density of 200/ha used.

Italian Wikipedia - Demografia di Roma: For Rome. Estimate of a population no higher than 30,000 after the end of the Gothic War in 554 AD. Bar chart gives a population of 40,000 in 563 AD.

Please note that this is in the middle of the Dark Ages, and that it's pretty much the most difficult medieval period to get population estimates for. Most of Europe at this time was in a deplorable state, which makes it nearly impossible to collect data for that region. This means that information here might indeed be subject to error, but there's unfortunately not much I can do about that.

Sources for new additions:

An Urban Geography of the Roman World: Lists an estimate of Justinian's walls for Antioch / Theopolis. While 400 ha is still large, the city would still be recovering from its ravaging a few years prior and thus a low population density. of 100 per hectares is used.

Settlement, Urbanization, and Population: For Salona (94 ha) and Oxyrhynchus. Population density of 200/ha is used for Salona. Article gives a population estimate of 20,000 to 25,000 for Oxyrhynchus during the Byzantine period.

Wikipedia - Homs: For Emesa. While it's uncertain how much the walls have shifted, it protected roughly the same area of 120 hectares during the Abbasid period. Population density of 200/ha used.

The Remains from Late Antiquity and the East Roman Periods and Their Location within the Lydian City of Philadelphia: New Comments: For Philadelphia. Walls protected an area of roughly 100 ha. Population density of 200/ha used.

3000 Years of Urban Growth: For Theodosiopolis. Gives 45,000 in 622 AD and 40,000 in 1200 AD. Missing in Four Thousand Years, thus estimate perhaps should be reduced to about 35,000?

The eastern frontier of the Roman Empire with special reference to the reign of Constantius II: For Amida. Area of (at least) 140 hectares. Population density of 200 per hectare.