Is it possible to detect if text is longer than one line in my div?

There is a jQuery plugin that may suit your needs: http://jedfoster.com/Readmore.js/

You can compare DIV width with scrollWidth to detect if text is overflowing:

if ($('.less')[0].scrollWidth <= $('.less').width()) {
   $(".text-size").hide();
}

Demo: http://jsfiddle.net/ygalanter/M2APS/50/