title: Make a div appear if another div is smaller than a specified size
body: <p>I have 2 divs, <code>.about</code> and <code>.hiddendiv</code>.</p>
  
  <p><code>.about</code> is an area for the site owner to write their 'about' info. but I have no way of knowing how much info they will put in there. So I want to do this, if <code>.about</code> is less than 500px in height then <code>.hiddendiv</code> will show otherwise it wont. I have no clue how to go about this, can anyone help?</p>