How to make font size bigger?

In the “Visual” section, H3 tag looks bigger than the H2 tag; the H2 tag looks bigger than then H1 tag. This makes sense.

However, when I publish the post, the H2 tag looks bigger than the H3 tag:

https://howtofocusbetter.com/test-page/

How can I increase my font size so that it looks bigger than the H2 tag?