Changing the font and font size is pretty easy in the PK4Connect email templates.

If you are using an saved email template then double click on the text in the email template. You see an option to change the Font and Size of the text.

If you are creating a new template, drag and drop the Text tile on to the template section from the righthand side tiles list and you see a small pop up to set the Font and the Size of the text in the template.