ok ho risolto così
codice:
<? if (!$col) { ?>
<script  language="javascript">
<!--
var s_width ='';
var s_height ='';
s_width=screen.width
s_height=screen.height

if (s_width == "1024"){ 
location.href="pagina.php?col=250"; 
} 
if (s_width == "1280"){ 
location.href="pagina.php?col=200"; 
} 
//-->
</script>
<? } ?>