Let's start with highest possible number - using the ancient Roman Empire. During the times of Roman empire, Rome peaked at 1-1.2 million at around 1st-3rd century.
Following that we had Constantinopole at 500 000 in 6th century, and Changan and Baghdad around 1 million pop in 8th, 9th and 10th century. 1 million remained cap until the 19th century.
Thus, I think we can conclude: 500 000 to 1 200 000 - maximum, imperial capital with extreme cultural significance to the empire (that is, empire needs to be capital-city centric). The empire needs to be large.
100 000 - 500 000
A capital city of a very prosperous kingdom. It is important to realize that number of people in a city can change depending. As an example, somewhat small Kingdom of Bohemia's capital city, during its golden era in 14th century reached 80 000-100 000. Larger kingdoms or smaller empire who are currently in their prosperous period will have around this amount of citizens, and so will large empires who're currently falling apart. Cities within 100 000 -300 000 range might also be secondary cities of an extremely prosperous empire, cities which hold sway over large region within such Empire.
Things like war, plague, etc. can massively decrease population of cities. In case of plague, cities tend to suffer a lot, and not only do you need to count victims, but also people fleeing the cities due to fear. When economy/food production is weak because of war, cities are also unable to feed their current population and people again leave.
50 000 - 100 000 population
This size would be a capital city of a kingdom that is currently in less prosperous phase. We're either talking about a small kingdom in quite prosperous phase, to a big kingdom in their bad times. For non-capital city, this could be a secondary city of a prospering kingdom, or a somewhat regular city of a very prosperous powerful empire.
20 000 - 50 000 population
These would be capitals of not too large kingdoms that are not too properous. If your kingdom is not the dominant power, and you're currently not in your gold era, this is the expected size of your capital. Example would be Vienna of 15th and 16th century(before Austrian and AH empires becaem a thing). As for non-capital cities, again, raise the bar of the country somewhat to at least a large kingdom during somewhat properous time.
Here, have a source for the largest cities of their time period and another one for large ones.