myWin = window;
YAHOO.util.Event.addListener(myWin,"load",testViewPortSize);

function activateLargeScreenCSS() {
    YAHOO.util.Dom.replaceClass("box", "smallScreenBox", "largeScreenBox"); 
}

function testViewPortSize() {
    viewPortHeight = YAHOO.util.Dom.getViewportHeight();
   if (viewPortHeight > 630) {
      activateLargeScreenCSS();    
   }
}