At the moment there is not this feature. You would need to modify the part of the script that creates the thumbnails, in the module file js/slideshowclasses.js
Modify line 180 from
this.thumbnailHolderInner.appendChild(this.thumbnails[i]);
to
var divcontainer = new Element('div', { 'class':'thumbnailDivContainer'});
divcontainer.appendChild(this.thumbnails[i]);
this.thumbnailHolderInner.appendChild(divcontainer);
This places the thumbnail inside a div container with the css class thumbnailDivContainer. You can then give this div element the css style
div.thumbnailDivContainer{ width: 80px; height:80px; overflow:hidden; }
obviously you change the dimensions to suit your needs.
I believe that should work.