Question is the title of this question...or do I have to use a custom field?
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It can't, 80 is max. Beats me why, my suspicion is it has something to do with UTF-8 encoding of characters (3 bytes / sign * 80 = 240 < 255 which is still a nice VARCHAR2 in underlying Oracle database).
You're more than welcome to use a custom field but I'm sure you're aware what's displayed in links to the record on all related lists, balloon pop-ups, search results etc ;)