How to change the wording in wp-admin back-end?

You can use the filter admin_post_thumbnail_html:

function custom_admin_post_thumbnail_html( $content ) {
    return $content = str_replace( __( 'Set featured image' ), __( 'Set featured image - 50 pixels by 50 pixels' ), $content );
}

add_filter( 'admin_post_thumbnail_html', 'custom_admin_post_thumbnail_html' );