There is no option for that.
You would have to edit the file "cart" of the view "checkout" via the menu Display->Views and change the line:
<?php 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:
<?php 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 size in pixel of your thumbnails.