You would need to change the eMail template that you are using - you can do this by doing the following:
From the Ribbon, click Options and then click Document Manager.
Click Manage Templates and edit the font style and size in the selected template.
Click Save and Close.
You might also wish to change the font style and size in your eMail signature which you can do as follows:
From the Ribbon, click Options and then click User Preferences.
Click Letter / Email and make required changes.
Click Save and Close.