Custom_field font size is not configurable. The font size works exactly like font sizing works with text fields assigned to signer roles. Font sizes start at 11px and automatically shrink as 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.