<div id="leftSideDataBox" style="height:auto; overflow:hidden">
// some web form contents
</div>

<?php $leftSideDataBoxHight=0;?>
<script>
     var myDiv = document.getElementById("leftSideDataBox");
     var boxHight=myDiv.clientHeight;
     <?php $leftSideDataBoxHight?>=boxHight;
</script>
<?php if(leftSideDataBoxHight<100)
{
//do some window arrangements
}?>

<script>
    var myDiv = document.getElementById("leftSideDataBox");
    alert(myDiv.clientHeight)
</script>

<div>
 // your div
</div>
<input type="hidden" id="hiddencontainer" name="hiddencontainer"/>

<script>
  var myhidden = document.getElementById("hiddencontainer");
  myhidden.value=boxHeight;
</script>

$boxsize=$_REQUEST["hiddencontainer"];
echo $boxsize;