Código PHP:
<?php
$url='<object width="640" height="385"><param name=movie value=http://www.youtube.com/v/TR5RQZ88P1U&hl=es_ES&fs=1&></param><param name=allowFullScreen value=true></param><param name=allowscriptaccess value=always></param><embed src=http://www.youtube.com/v/TR5RQZ88P1U&hl=es_ES&fs=1& type=application/x-shockwave-flash allowscriptaccess=always allowfullscreen=true width="640" height="385"></embed></object>';
$busqueda = array ('@width=".*?"@si','@height=".*?"@si');
$reemplazar=array('width="300"','height="500"');
$url = preg_replace($busqueda, $reemplazar, $url);
echo "<pre>";
echo $url;
echo "</pre>";
?>
Prueba aquí:
http://www.linkalis.com/galeria/expresiones.php