Thanks for making me look for solution to this problem;

(Instructions for Windows XP)
BEWA This will change the TOOL TIP appearance everywhere on your system...

1. Right-click on the desktop, and choose Properties
2. On the Appearance tab, click Advanced.
3. From the Item dropdown, choose Tooltip
4. 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)

BONUS: Visit http://www.contextures.com/xlcomments02.html#Default
I got the above from this page
"andy f" wrote:

Advice i've found so far relates to XP and is not Vista friendly