So any size transparent thingy will do right?
Size of transparent gif can be set via
/includes/languages/english/nameofyourtemplate/header.php
i.e.
Code:
define('HEADER_LOGO_WIDTH', '300');
define('HEADER_LOGO_HEIGHT', '75');
define('HEADER_LOGO_IMAGE', 'logo.gif');
but 1pix x 1 pix will probably be fine