What is PMC Page Builder? If it is a paid plugin or theme, you should contact the developer for support.
Generally font sizes in websites are determined by 1) the browser settings and 2) the CSS stylesheet of that site. For WordPress powered sites the stylesheet is part of the theme.
To edit the stylesheet, create a child-theme (learn more about ihow to do that here – http://codex.wordpress.org/Child_Themes) and then edit your child-theme’s stylesheet to change the font sizes as needed. Browser developer tools are invaluable for this – I particularly like Firefox’s plugin Firebug for figuring out what is applying what style and where on the page.
Another way to edit you theme’s stylesheet would be to use a custom css plugin (https://wordpress.org/plugins/search.php?q=custom+css) to add your own custom styling as needed.
Hope this helps.
