isotope/_includes/random-sizes.js

      // add randomish size classes
      $container.find('.element').each(function(){
        var $this = $(this),
            number = parseInt( $this.find('.number').text(), 10 );
        if ( number % 7 % 2 === 1 ) {
          $this.addClass('width2');
        }
        if ( number % 3 === 0 ) {
          $this.addClass('height2');
        }
      });