Custom Certificates do not have the ability to change the font size, color, nor type. Any additional text that is not part of the drop down options, which remain constant, can be added to the background image of the Custom Certificate. There are 2 different font sizes in the Custom Certificates. In the center stack of fields, the font size is 24pt.
The bottom row of fields are 15pt.
Custom certificates are available only on the Enterprise plan.