getting width of a div element

HTML:

<a id="postanad"  href="postad.php">Make a Post</a>

CSS:

#header #postanad {
    position: absolute;
    top: 50px;
    left: 250px ;
    background: #000 url(postad.gif) repeat-x 0% 100%;    
    font: bold 19px Tahoma, 'trebuchet MS', Sans-serif; 
    padding: 5px;


}

How do I determine the size in pixels/ems that the div element is taking so that I can make the background image postad.gif to be of the same width?

Just to be clear I don't want to determine the width dynamically , but just want to know how much space it is taking so that I can draw a background image in PS.