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The font size is in fact hard coded into the Cufon functionality. In your theme directory navigate to theme/includes/js/cufon-customization.js and you will see in there:
Cufon.replace('#site-title', { fontFamily: 'Gnuolane' });
Cufon.replace('#navigation a', { fontFamily: 'Gnuolane', fontSize: '15px' });
Cufon.replace('#footer-navi a', { fontFamily: 'Gnuolane', fontSize: '13px' });
Cufon.replace('h1, h2, h3, h4, h5, h6', { fontFamily: 'Gnuolane' });
Cufon.replace('#sidebar h3', { fontFamily: 'Gnuolane', fontSize: '15px' });
Cufon.replace('#site-description', { fontFamily: 'Vegur' });
Change the (‘#navigation a’) fontSize to whatever size you’d like:
Cufon.replace('#site-title', { fontFamily: 'Gnuolane' });
Cufon.replace('#navigation a', { fontFamily: 'Gnuolane', fontSize: '20px' });
Cufon.replace('#footer-navi a', { fontFamily: 'Gnuolane', fontSize: '13px' });
Cufon.replace('h1, h2, h3, h4, h5, h6', { fontFamily: 'Gnuolane' });
Cufon.replace('#sidebar h3', { fontFamily: 'Gnuolane', fontSize: '15px' });
Cufon.replace('#site-description', { fontFamily: 'Vegur' });