Thanks, @Italyuser . I finally worked out that you were showing me a link to an old answer.
Wow, what a well-hidden feature. I have no idea how you would find this by searching the manual, or learn about it other than in a forum such as this.
I note that this seems to work only within the current project, and must be repeated for all projects. I think.
It also doesn’t appear to work “globally” for lots of other symbols, which still must be done one at a time.
Here’s what @dspreadbury said some time ago…
"Open the Engraving Options dialog, go to the Chord Symbols page, and scroll to the very bottom, and click ‘Edit’ to open the Project Default Chord Symbol Appearance dialog.
In the dialog that appears, enter a minor chord in the box at the top-left corner of the dialog, e.g. Cm. Click the + button and you’ll see the chord symbol shown in the main editor area on the right. Select the “m” sign component of the chord symbol in the editor. The “m” sign component will also be shown as selected in the grid of components below the editor. Click the pencil icon to edit this component.
A second dialog opens: in here you can edit the appearance of the "m" sign to be however you want it to look, e.g. increase its scale factor, or use a different font character.
Confirm this dialog and its parent. Now whenever you have a minor chord, it will use your edited component."