Custom_field font size is not configurable. In fact, the font size works exactly like font sizing works with text fields assigned to signer roles (so it starts out at about size 11, and will automatically shrink as the text becomes too large for the space provided).
During template preparation, assigning a text or checkbox field to “Me (when sending)” creates a custom_field. Custom_field data can be entered when sending a POST signature request with template call via the API or when sending a signature request from hellosign.com using a template
*Please note that hellosign.com usage may require a paid hellosign.com subscription and is not explicitly included with a HelloSign API subscription