Enabling Extra Plugins - *.ckeditor5.yml

In your MODULENAME.ckeditor5.yml file:

MODULENAME_fontsize:
  ckeditor5:
    plugins: [ckeditor5-font.FontSize]
  drupal:
    label: Font Size
    library: MODULENAME/fontsize
    toolbar_items:
      fontSize:
        label: Font Size
    elements: false