I have researched the Droid Incredible and I am showing that the font setting can only be changed in the email settings, and not for SMS or MMS messages. You may want to try a messaging application from the Android Market and see if the application has this setting for SMS/MMS messages. Below I have provided you with the steps on how to change email font without having to download anything from the Market.
Press HOME , and then tap > Mail.
Switch to the email account you want to edit.
Press MENU and then tap More > Settings.
Select General Settings, then press Font
Select the Font and press OK or Save.