<?php
function update_medium_large_size_wpse216595() {
    add_image_size( 'medium_large', 600, 255, array( 'center', 'top' ) );
}
add_action( 'init', 'update_medium_large_size_wpse216595', 11 );

add_image_size( 'medium_large', 600, 255, array( 'center', 'top' ) );

add_image_size('medium_large', 600, '', true);