Вообще-то и id не очень нужен...
<!DOCTYPE html>
<html>
<head>
<title>Radio</title>
</head>
<body>
<div id="sel">
<input type="radio" name="size" value="[size=20pt]" />[size=20pt]
<input type="radio" name="size" value="[size=20]" />[size=20]
</div>
<textarea id="result"></textarea>
<script>
var result = document.getElementById('result');
document.getElementById("sel").addEventListener('change', function (e) {
result.innerHTML = e.target.value
})
</script>
</body>
</html>