Default text size, font, and formatting is now available in Gmail via. Google Labs.
Just log into Gmail, go to “Settings” and click the “Labs” tab, and then locate “Default Text Styling” by Johnathan K.
“Would you prefer Times New Roman to Arial, or perhaps large text to small text, in the e-mails you send out? Enable this lab, head over to the ‘General’ tab on the ‘Settings’ page, and change the default text styling to better suit your preferences.”
The official post can be seen on the Official Gmail Blog
I've been waiting forever for this!