Fontsize
Change the font size specified in pixels. Bigger sometimes is better.
Code
<!DOCTYPE html>
<html>
<head>
<title>Redactor</title>
<meta charset="utf-8">
<!-- redactor css -->
<link rel="stylesheet" href="/your-folder/redactor.css" />
</head>
<body>
<!-- element -->
<textarea id="content">...</textarea>
<!-- redactor js -->
<script src="/your-folder/redactor.js"></script>
<!-- plugin js -->
<script src="/your-folder/plugins/fontsize.js"></script>
<!-- call -->
<script>
$R('#content', { plugins: ['fontsize'] });
</script>
</body>
</html>