This tutorial is designed for Doc2Form version 3 but steps will be similar for other versions.
Luckily Doc2Form uses an ascx file for most of it's programming so it is easy to make changes to it. In order to change things like the font size of emails that are emailed by Doc2Forms "Doc2Form - Email (no save)" macro - and others, follow the steps below:
Now reload your email form and test it. The font should be the standard for the page. You can of course make changes like this in other areas of the page, for example adding a style attributes to the div tags to make more detailed changes.