To change the font size of the blog titles, when on the code editor, search for the file "article-template".
On the h1 tag, add the following inline style, style="font-size:34px".
This will guarantee that it will override the other font sizes.
The h1 tag is on line 28 of version 4.10.2, if you are on a different version, it may vary.
Please find the screenshot as a reference of where you should add the code.