Playing around with this at the moment.
Works after a fashion.
Please change size to suit
Best maybe to keep original ratio size of cell.
So check computed values in Fast Dial/Preferences/General/General/Size
before changing values in code
@namespace url(http://www.w3.org/1999/xhtml);
@-moz-document url("chrome://fastdial/content/fastdial.html") {
}
div.thumbnail:hover
{
width: 480px !important;
height: 360px !important;
}