Configuring preferences and options in Outlook.com
In Outlook settings, you can specify that you want to use HTML as your preferred e-mail format, and you can also specify which font and size you want to use by default. The screen shot below shows the Settings dialog box for Outlook on the web, but the new Outlook for Windows looks similar:
Existing mails that you have already sent or are in the middle of composing won't be affected when you change the default settings, though.
For an e-mail that you are currently composing, you can change the format from the Options tab. Click Switch to HTML (if necessary) and then use the controls on the Format Text tab to modify the appearance of the text in the e-mail.