hmm do you edit the right css file? since without the url we can't go forward.
But there another way to do it without edit style sheet(not recommend)
go to your language folder find the "tagline here" then you can use HTML code to change the size of the text simple like
Code:
  define('HEADER_SALES_TEXT', '<font size="20">Tagline</font>');
or

Code:
  define('HEADER_SALES_TEXT', '<font style="font-size: 4em;">Tagline</font>');