Best font size for both desktop and mobile users

I am creating a series of courses and I'm struggling to determine what font size is best for a course that will be accessed from computers and mobile devices. Right now I have the font size at 16-18 depending on the content as that is what seems to look best on the mobile, but it seems kind of small for a desktop.

We suspect that 75-80% of our users will be accessing the course from a mobile device, so if nothing else, I will lean towards what works best for mobile display knowing that those on a computer can increase the zoom settings on a browser.