(This is if you want every message to have the same font type by default)

In OE, go to TOOLS / OPTIONS /

Click on the COMPOSE tab.

Under COMPOSE FONT / MAIL, click on FONT SETTINGS.

 Now just change the font style, size, and color here.

OK that box, then APPLY / OK the other box.

Now every e-mail will use these by default. However, you can still change a message settings individually as you compose it. Changing the settings in an indivual message won't change the default.