How does the font of Apple SMS get bigger?

Take the iPhone 7 mobile phone as an example, the method of changing the font size is as follows:

First, click "Settings" on the desktop of iPhone 7.

Second, select the "Display and Brightness" option after entering.

Third, then find the item "font size" and click enter.

4. After entering, slide the slider in the red box below to the right to enlarge the font.