Do you really need to use iframe? This works for me:
CSS:
#Descriptiondiv {
width:80% !important;
height:250px;
display:none;
}
HTML:
<div id="Descriptiondiv"> CONTENT HERE </div>
<a href="#Descriptiondiv" rel="prettyPhoto">link</a>
You need to use the !important or else the prettyPhoto will use the default width (500).