The font size of text elements within your website can be scaled up or down using these settings. Notice that font sizes are specified in percentage increments. This is done to improve browser compatibility and enable people visiting your website to scale text up or down in size based on their web browser settings.
You can apply your own font size settings to individual elements using custom CSS. Here is an example of what the CSS code may look like:
#selectorName { font-size: 95%; }
Typically this code should be added to the Custom CSS box under the page inspector.