Yes, you can easily change the font size. The font size is managed by each printer location you setup. WooCommerce > Print Manager > Printers tab. Click add or edit to view the settings for the printer location.
If you have selected the Output format of HTML, as shown here: https://ibb.co/sqfp0Kk
You can manage the font size of various elements of the template.
If you have selected the Output format of Plain Text, as shown here: https://ibb.co/TRgY0NZ
You manage the font size based upon width of the paper, the font will adjust based upon the width size.
For Template customization, we offer several no code options for adding a logo, company information, section information and footer information.
Beyond these options, we offer full customizations of templates with our development information and ability to create a fully customized templated based upon all your requirements on design, style, layout and included order information. You can learn more here: https://getbizprint.com/documentation/developer/