Changed font size in featured slider for mobile use

Hi there, I changed the featured slider font size putting additional css: #main-slider .slides .entry-title a { color: #fff; font-size: 34px; } This now also stays with small screens, guess I need additional css for separate screen sizes? This must have asked many times before but I couldn't find a soltion in the forum entries so far. Is there css out there that I could use: http://www.stiftung-meeresschutz.org/ Thanks Regards Ulrich