To change the default font size (this will also change the settings for
tooltips in Excel and other programs):
Right-click on the desktop, and choose Properties
On the Appearance tab, click Advanced.
From the Item dropdown, choose Tooltip
Choose a font Size, click OK, click OK
(Note: selecting a font type here will not affect the default font used
in Excel's comments)
To insert a comment with a specific font, you can use a macro:
http://www.contextures.com/xlcomments03.html#Formatted