Most of this article has focused on the ideal blog font size with the only goal being optimizing user experience.
However, if you have display ads on your blog, you are also trying to make money, while balancing this user experience.
Based both on personal experience across my 10+ websites, as well as the recommendations from top ad networks, I can say with near certainty, that blog font size should be at least 18 pixels.
I currently use 20px for my blog paragraph font (I started at 16px, increased to 18px, and more recently increased to 20px)
This was after reading Mediavine’s guide, which suggested experimenting with increasing font size:
Again, your main body size should be at a minimum 16px…We’d actually recommend going a little bit bigger than the minimum 16 px, just to be on the safe side, given differing devices: 18px should do the trick. That may sound like a big number, but remember where your audience is reading, and that it’s not an 18pt font; 18px is actually still pretty small on a mobile device.
They go on to explain how many of their top performing blog use a main body text size of 20px.
I personally saw a 20%+ increase in my site’s RPM, just by increasing the font! This was literally in a matter of days!
The point is, you should experiment with larger font sizes, but based on the current research, ad industry guidelines, and my own personal experience:
Your blog’s font size should be 18px or 20px for main body text in your blog. And with increasing use of mobile devices, even if you set your website’s desktop font size to 18px, I’d highly recommend trying 20 px for mobile devices, as it is much easier to read.