In Western Europe, of cities that had existed during the empire during the 4th century, which grew in population during the 5th and 6th centuries: none, notta, zippo.

The reasons for the existence of many Roman cities and the maintenance of their size, was because they were administrative and consumption centers.

They depended upon steady access to the Roman exchange network, which requires a class of specialists to make goods for trade or consumption by elites along with their servants and clients. It also needed a high degree of functioning bureaucracy to extract taxes from the peasants, and a capable military funded well enough to protect all of those within the city.

In the 5th and 6th century, all of those aspects were in free fall in western europe, leaving the cities left barely existing as administrative areas, but certainly not major of trade, specialist production or luxury consumption that could account for the same city population size as in the 4th century.

tl;dr - "Cities" were lucky to exist at all. None of them were growing and I highly doubt as well any were of the same size as in the 4th century.