You should edit the file "cart" of the view "checkout" of your frontend template via the menu Display->views and change the code:
echo $this->image->display($image->file_path,true,$image->file_name,'style="margin-top:10px;margin-bottom:10px;display:inline-block;vertical-align:middle"');
to:
echo $this->image->display($image->file_path,true,$image->file_name,'style="margin-top:10px;margin-bottom:10px;display:inline-block;vertical-align:middle"','',XXX,YYY);
Where XXX and YYY are the width and height for your thumbnails