Dr.Joel,
IF you are using OL2000 (you really should have specified):
menu Tools>Options>Mail Format>Click the Fonts button>Select "When composing
and reading plain text" and set the font size.
This will only affect Plain Text formats. To affect ALL incoming messages,
you must also select "Always use my fonts' in the lower box. You should also
set the fonts for composing, replying and forwarding in the upper section.
It is highly recommended that you choose a universally used font such as
Arial or Times New Roman. Otherwise, the recipient of your message may
substitute a font that ruins the appearance of your message (their computer
will substitute without user involvement).