The city size of Civilization IV will always be 1.

First of all, you have to see where the citizens of your city work. Is it in places where there is no grain output, such as gold mines and loess hills, that one population consumes two grains? If a citizen does not work in a place where there is no food output except two kinds of food produced by the city itself, then there is no surplus food to increase the population. Therefore, the primary task of a city is to let the first citizen work in a place with high food.

Secondly, the population will not increase when the city builds immigrants and workers. At this time, the surplus grain will be transformed into additional industrial value, thus accelerating the construction of immigrants and workers.

Another factor is that you turned on the city control button to control the population growth. In this way, even if food is abundant and the population growth column is full, the population will not increase.

But according to your description, if it is 0/ how much, it should be the first case. You have to click into the city, and then put the circle representing the citizens where there is a lot of food. If you are a novice and don't know how to operate, there is a button in the lower right corner of the city page to automatically control the population distribution. Click it and let the computer help you configure it. In general, the configuration is good.