Fijate si te sirve:
Código PHP:
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1" />
<title>test</title>
<script>
function $(id){
return document.getElementById(id);
}
function addEvent(obj, evType, fn, useCapture){
if (obj.addEventListener){
obj.addEventListener(evType, fn, useCapture);
} else if (obj.attachEvent){
obj.attachEvent("on"+evType, fn);
} else {
obj['on'+evType]=fn;
}
}
var reng=5;
function agregar(){
$('quita').disabled=false;
reng=reng+5;
$('pp').rows=reng;
}
function quitar(){
reng=reng-5;
if(reng<6){reng=5;
}
$('pp').rows=reng;
}
window.onload=function(){
addEvent($('quita'), 'click', quitar, false);
addEvent($('agrega'), 'click', agregar, false);
}
</script>
</head>
<body>
<form id="form1" name="form1" method="post" action="">
<textarea id="pp" name="textarea" cols="60" rows="5"></textarea>
<input id="agrega" type="button" name="Submit" value="+" />
<input id="quita" type="button" name="Submit2" value="-" />
</form>
</body>
</html>