This request can be closed and marked resolved. It was an urgent matter so I searched for a plugin to replace the old blog page.
For anyone having an issue with the formatting of their blog page or author byline problems, the free plugin Blog Designer will probably be a good choice. Just look for it and install it in WordPress via the dashboard.
This problem has been resolved.
The issue was resolved in a theme update, so the font size is now corrected in the most recent version.
