You can change the view zoom setting in your browser which will change the font sizes or you can adjust the printing format.
It uses a style sheet so class=invoice will inherit the standard font sizes we use but you can use any HTML formatting command.
e.g. <td style=”font-size: 11pt”>..</td>
You would need to insert in below 2 sections the company details and address window. You would also need to put it anywhere else if you wanted to change the entire template and class=invoice