which version of Outlook?
I don't think the default is part of any global setting but in Outlook 2007
you can certainly change the font on the fly via the Format Text Ribbon or
via the neat little pop-up thingo when text is selected.
I'm not speaking of items in the Notes folder, I'm speaking of the notes
pane in Contacts. I tried it before posting, so I know it works, at least
for Outlook 2003. Since the OP didn't state his or her version of Outlook,
I chose Outlook 2003.