[Deff]

<script src="http://code.jquery.com/jquery-latest.js"></script>



<form action="#" method="post" name="">            
            <select name="size" id="size">
                      <option data="disabled" value="значение">disabled</option>
                                    <option value="значение">значение</option>
                                    <option value="значение">значение</option>
                     <option data="disabled"  value="значение">disabled</option>
                                    <option value="значение">значение</option>
                                    <option value="значение">значение</option>
</select>
<input disabled type="submit" name="resize_download" value="ok" class="button" />
</form>


<script type="text/javascript">
$('#size').change(function(){
  if( $(this).find("option:selected").attr("data") == "disabled") {
     $('input[name="resize_download"]').attr('disabled','disabled');
  }  else $('input[name="resize_download"]').removeAttr('disabled');
});
</script>