Hight of layer

[wxdqz]

<div id="Layer1Div" style="position:absolute; width:150; height:40; visibility:visible;">
Michelle
</div>

When I use the above layer, I can get the height of it using this code:
alert("height: "+document.getElementById(Layer1Div).style.height);

When I have the layer below, notice all the <br>, the actual size of the layer is not 40, it is actually more on the screen.

<div id="Layer1Div" style="position:absolute; width:150; height:40; visibility:visible;">
Michelle
<br><br><br><br><br><br><br><br><br>Michelle
</div>

How can I get the real size of the layer?

Any thoughts?
// michelle