How to display a select dropdown

Have looked everywhere and I can’t find any CMB2 documentation that outlines how I can get the following to display on the front end. Does anyone know how to echo out a simple select dropdown to the front end, thanks?

      add_action( 'cmb2_admin_init', 'cmb2_select_size_metaboxes' );

      function cmb2_select_size_metaboxes() {

        $cmb = new_cmb2_box( array(
            'id'            => 'size_metabox',
            'title'         => __( 'Select Size', 'cmb2' ),
            'object_types'  => array( 'product', ), // Post type'context'       => 'normal',
            'priority'      => 'high',
          'type'             => 'select',
            'show_option_none' => true,
            'default'          => 'custom',
            'options'          => array(
                'standard' => __( 'Option One', 'cmb2' ),
                'custom'   => __( 'Option Two', 'cmb2' ),
                'none'     => __( 'Option Three', 'cmb2' ),
            ),
        ) );
      }