Lets get right to the point; a font size of 16 is best all around and here are some standard practices we do on our mobile sites.
Along with the font size of 16 we do not use H1, H2, H3, H4 font sizes. We will use the font size of 16 and make it bold or use a font size of 18-22 for a header text.
Keep in mind that mobile visitors to your site are looking for information quickly and interaction, not the eye candy of your site. So keep it simple with a simple easy to navigate and easy to read mobile site. Your visitors will love you for it.
Under divi customizer you can set font size under mobile styles. Setting this to 16 will help you a lot with this default setting.