ecco a te....

Codice PHP:
<html>
<head>
<title>PROVA</title>
<script language="javascript">
<!--

function redim(valore){
  var len=valore.length;
  if(len>=15)
    document.myForm.myText.cols=len+1;
}

//-->
</script>
</head>
<body>
<form name="myForm">
<textarea name="myText" rows="1" cols="15" scrolling=no onKeyPress="redim(this.value)">ciao ciao</textarea>
</form>
</body>
</html>