Thanks, Marc V. for the bugzilla link. I’ve added an amendment to that bug which should make a separate “resize the cardnumber” bug very simple (Yes, +1 to the idea of increasing its size). Feel free to test bug 15690. Look at a preference and run a prove.
Not very hard.
https://bugs.koha-community.org/bugzilla3/show_bug.cgi?id=15690