Component Editor (VCL)
You can set IconFontsImageList properties using the classic Delphi Object inspector, in combination with the collection editor and the Structure editor, but it's most simple to use the advanced Image Editor supplied: double-click to the component to open the component editor as you can do with any Delphi component.
In this figure you can see the advanced component editor for VCL version used by the IconFontsImageCollection component:
And the same component editor into dark IDE:
At the left you can see and select categoriries of the Icons.
Under the Images/Categories list, you can set the global properties of your IconFontsImageList or IconFontsImageCollection instance.
At the center of the editor you can see a preview of the icons in the collection. Selecting an icon you can see the IconFontsImageListItem properties and change it.
This is the same component editor used by a IconFontsImageList or IconFontsVirtualImageList: notice the Top panel with the "size" properties of the icons.
You can also specify a different Font on every single icon, look at this example: the default fontname is "Font Awesome 5 brand regular" to render used to render the forst 10 icons, then the last 2 icons are rendered by "Material Design Icon" font.
In this editor you can also Import Icons from a Font using the Custom Char Map made with IconFontsImageList itself.
Using "Export" button you can save a bitmap with all the icons of the ImageList, if you want to use them with a normal ImageList or for other scopes.