Hi!
I found and tested this script. You can paste it into the custom code or in a separate js-file. Appoint id for text area and paste it in the script.
function flexibleTextarea(){
var _txt = document.getElementById('comment'); // comment is id of text area, that you want to expand
var _minRows = 7; // height of field (number of rows)
if (_txt) {
function setRows() {
_txt.rows = _minRows;
do {
if (_txt.clientHeight != _txt.scrollHeight) _txt.rows += 1;
} while (_txt.clientHeight < _txt.scrollHeight);
}
setRows();
_txt.rows = _minRows;
_txt.onkeyup = function(){
setRows();
}
}
}
if (window.addEventListener)
window.addEventListener("load", flexibleTextarea, false);
else if (window.attachEvent)
window.attachEvent("onload", flexibleTextarea);