An alternative way to achieve this result is to use a message-field and insert html with font size and style that you desire.
<html>
<body>
<span style="color:#333333"><B>USAGE FROM</B></span>
<B><span style="color:red;font-size:16pt">$(MIN_DATE, date, MMMM d)</B>
<B><span style="color:#333333">TO</span></B>
<B><span style="color:red;font-size:16pt"> $(MAX_DATE, date, MMMM d)</span></B>
</body>
</html>
Change the "rich-text-type of the message-field to "text/html" and that will give you the desired output.