I went to sysconfig and went to the Groups and told it to show Ticket. Then I looked at all the Frontend::Agent categories and none of them have anything for customerinfo.
The first option that could fit your needs is called "Ticket::Frontend::CustomerInfoComposeMaxSize" and is by default set to "22" signs.
Do a search for the word "customerinfo" in the Sysconfig Startpage ... you should only find the Ticket::Frontend section.