Is there a possibility to enlarge the letter type in e-mails via CIC 3.0?

It depends on the types of emails you're talking about. Voicemails and call recordings are sent using HTML templates that are in the Resources folder. The tags set the font and size, and you can change them in the HTML template to apply to the messages. Other messages that may be sent not using HTML would need to be updated (in handlers, most likely) to use a similar HTML template allowing you to set the size the same way.