Changing the font size in a WordPress header requires editing specific files. The precise code to edit will vary according to your WordPress theme, but every theme has a stylesheet containing the ...
To override the default Header Size and Content Width settings on a specific page, navigate to the bottom of the page you want to customize, and expand “Template Settings” in the righthand options ...
Google's John Mueller said that the size of your header tags, i.e. H1s, H2s, etc, does not matter for SEO or ranking purposes. John said it might matter for users and conversions but for SEO, nope.