Give this a try and see if it works for you. I've tested it on both iPhone and iPad, and it should function properly. Make sure to test it out on your own website as well.
<!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=utf-8" />
<meta name="apple-mobile-web-app-capable" content="yes">
<meta name = "viewport" content = "initial-scale = 2.3, user-scalable = no">
<title>Untitled Document</title>
<style>
body {
width: 100%;
}
textarea {
width: 97%;
padding: 5px 0.5%;
height: 5em;
line-height: 1.5em;
border: 1px solid #ccc;
}
</style>
</head>
<body>
<textarea>
</textarea>
</body>
</html>