in the webassist/email/templates folder, open the template fole for your email.
you will need to edit the inline styles for the elements to change font zise and color.
for example the email header code will look like this:
<h1 style="padding: 0px; margin: 0px 0px 2px 0px; font-size: 16px; text-decoration: none; font-weight: bold; color: #666;">Your Email Title Goes Here</h1>
you can edit the font size:
font-size: 16px
to something bigger:
font-size: 22px
or color:
color: #666;
to black:
color: #000;