There are two different places you can change your Font size, and one can override the other.
CASE 1. You can change it in Customize Design.
For example to change the Page Introduction you can click on the Page Introduction menu
This will bring up a dialog to edit the text such as below
This is showing my text should appear in Green and Font size is 14px. But from my example you can see that the text is Blue.
So now you need to check the other place you can change the text (which is over-riding this font size and/or color)
CASE 2. You can change the font size with HTML directly in the page Introduction in Normal view
In the window above HTML was used, so if you click on the texts in the above window, close to where you can see the red text, you will see the HTML as shown below.
Depending on the theme you have used and what changes you have made. One setting overrides the other. In this case, the color setting is coming from the HTML (not Customize Design) and the size is coming from Customize Design (14px).
If you want your text to appear at size 5 as per above, you need to go into Customize Design and remove 14px from the text size