Auto size (height) for <textarea>

autoareasize.js

;(function() {

  var sham = $('<pre></pre>');

  sham.appendTo('body');

  $(document).on('input change focus', 'textarea', function() {
    var textarea = $(this);

    sham.css('width', textarea.outerWidth());
    sham.text(this.value + '&nbsp;');
    sham.attr('class', 'textarea-sham ' + textarea.attr('class'));

    textarea.css('height', sham.outerHeight());
  });

})();

autoareasize.scss

// Add this to your stylesheet (SCSS required)

.textarea-sham {
  @extend textarea;

  display: none;
  height: auto;
  white-space: pre-line;
}