If you'd like to change the default font size in the Divi Theme, you can do so by adding CSS to Divi, like so:

body { font-size:130% !important; }

This will change the font-size of many components, such as there text in fonts, in the footer and in the sidebar. Note that elements which override the default font-size (such as the links in the header) will not be affected by the font size change.