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. C-. Click the + button and you’ll see the chord symbol shown in the main editor area on the right. Select the minus sign component of the chord symbol in the editor. The minus 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 minus 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 that uses the minus sign, it will use your edited component.