>>96519654
your best bet is actual historical research about population density. Then picking a time period that matches your setting as far as tech level/ societal and population decline goes etc. Finally pick a place/nation with similar geography to the region you want to develop.
https://sourcebooks.fordham.edu/source/pop-in-eur.asp
this is one source i found with a simple google search.
In europe, which is the place i am familiar with, for a place like scoland, population density was around 4 people per sq mile around the 1000s. A 5 mile hex is around 31 square miles so we get around 125 people per hex. if this clan has 160 hexes under their lands this means somewhere around the 20.000 people mark.
You should note that most places were far more populated than scotland.
Northern france has around 36 people per square mile so this territory would have 9 times the population, so around 180.000 thousand people.
Do not that by the 1300s the population has doubled.

Now to the second question of the size of independent city states and nations, once more check historical examples. The city state of Florence for example controlled around 1/3 of the region of Tuscany in the 1000s which boils down to around 95 5 mile hexes