You can change the default font size, but not the default font type.

--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)
You can also set the font to Bold or italic, and select a colour

--To insert a comment with a specific font, you can use a macro:

http://www.contextures.com/xlcomments03.html#Formatted

Juliana wrote:

--
Debra Dalgleish
Excel FAQ, Tips & Book List
http://www.contextures.com/tiptech.html