Just below the email's subject line, there is a formatting menu that allows to edit the font -- including the font type, size, and style.
To change the font size:
Any text you selected changes to the new font size. Also, if you type new text, it appears in the new font size.
Note: If you don't see the formatting tab, it is because you are in plain text mode. To switch to rich text mode, click the double-arrow button beneath the subject line.