The headings font size is configurable in the theme options, but there is only one setting which applies to all headings (as they have synchronized sizes).
If you want to customize particular headings, use CSS:
Hi,
Sorry, I know, the subject is old, but it concerns the Hn.
What is “default style sheet for HTML 4” for this site? https://www.w3.org/TR/CSS22/sample.html