cambiar valor de width

[juan_14nob]

Código HTML:

<script language="JavaScript">

if (screen.width<1024)
   document.write ("Pequeña")
else{
   if (screen.width<1280)
      document.write ("Mediana")

   }else{
   	   var tit=document.getElementById('header');
		tit.style.width ='50%';
   }
      

</script>