The font size of the links in the main and secondary menus may be changed for site design preference, and this is done in the site's theme.
The main navigation menu carries the exposed subpages, while the anchor links in a subpage are rendered in the secondary navigation menu. When there is only one subpage, the anchor links, if any, may be placed in the main navigation menu.
To change the font size of the links for main and secondary navigation menus:
1. In the Themes dashboard, hover over the theme you wish to edit, and click the Edit button on the right. The theme might have to be cloned first before applying any changes into it. You can refer to this article about how to clone a theme.
2. To change the font size of the menu link for the main navigation, expand the Navigation under Site Header from the left pane of the theme editor, and change the Font Size value. To change the font size of the menu link for the secondary navigation, expand the Submenu under Site Header from the left pane of the theme editor, and change the Font Size value.
3. Click the green Publish button on the upper left.