How to set panel icon size?

Desktop
1

What other value style_class has ?

let gicon = Gio.icon_new_for_string(this.dir.get_path() + '/gnome.svg');
const icon = new St.Icon({
    gicon: gicon,
    style_class: 'system-status-icon'               
});
this.indicator.add_child(icon);
2

If you are looking for the style class names for panel icons, look at the _panel.scss.

system-status-icon is the correct style class name if you are using it for the quick settings indicator.