Get Image Width From WP_Customize_Image_Control() File Set in WP Theme Customizer

Modern answer to this question:

We have WP_Customize_Media_Control control in WP since 4.2 that gives you the attachment id.

This answer was posted on I similar question.
You can see the documentation for WP_Customize_Media_Control here.

Exemple of use:

$wp_customize->add_setting( 'my_menu_image' );
        $wp_customize->add_control( new \WP_Customize_Media_Control(
            $wp_customize,
            'storms_menu_image_ctrl',
            array(
                'priority'    => 10,
                'mime_type'   => 'image',
                'settings'    => 'my_menu_image',
                'label'       => __( 'Image on menu bar', 'my' ),
                'section'     => 'title_tagline',
            )
        ) );

When retrieving the info, you can do this:

$image_id = get_theme_mod( "my_menu_image" );
if ( ! empty( $image_id ) ) {
    $url          = esc_url_raw( wp_get_attachment_url( $image_id ) );
    $image_data  = wp_get_attachment_metadata( $image_id );
    $width = $image_data['width'];
    $height = $image_data['height'];
}