[Trying to collate all relevant info, partially for future reference, but also for a discussion about username length...
Tentatively/contemplating proposals to:
a) lower the MediaWiki default - it's //very// large, and would make history pages (etc) even harder to read
b) re-examine the Wikimedia default - perhaps to reduce back down to 64, or override new-account creation to 64 limit if possible.]
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* MediaWiki (software) defaults to a maximum username length of 255 //bytes//, per https://www.mediawiki.org/wiki/Manual:$wgMaxNameChars
* Wikimedia's installation currently limits usernames to 85 //bytes//, since April 2015, per https://phabricator.wikimedia.org/rOMWC8b22946638841c93d0bee24793d10ed6b8ff7080
** E.g. https://en.wikipedia.org/wiki/User:A_wonderful_serenity_has_taken_possession_of_my_entire_soul,_like_these_sweet_morning
** The increase from 64->85 was to allow usernames already at the 64 byte limit, to still log-in after SUL auto-renaming, if their home wiki was e.g. zh_min_nanwiktionary (20 bytes + ~)
Historically...
* There was a limit of 40 characters at Enwiki, until: [[https://en.wikipedia.org/w/index.php?title=MediaWiki:Titleblacklist&diff=623436216&oldid=620873046 |2014 removal]], [[https://en.wikipedia.org/wiki/MediaWiki_talk:Titleblacklist/Archive_3#Why_are_usernames_of_more_than_40_characters_disallowed.3F| 2014 discussion]]
* There was a limit of 40 characters at Commons, until: [[https://commons.wikimedia.org/w/index.php?title=MediaWiki:Titleblacklist&diff=164730250&oldid=163996083| 2015 removal]], (older [[https://commons.wikimedia.org/wiki/MediaWiki_talk:Titleblacklist#username_length_restriction|2012 discussion]])
* There was a discussion at Meta, about making the length limit a global standard, but no consensus: [[https://meta.wikimedia.org/wiki/Talk:Title_blacklist/Archives/2013#Username_length_restriction |2014 discussion]]
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For reference, here are some **existing example usernames, and their [[https://mothereff.in/byte-counter|bytecount]], for actual editors** (please add a few more!)
**ascii**:
* 33: `Chase me ladies, I'm the Cavalry`, (and a few others at 33, in [[https://en.wikipedia.org/wiki/Wikipedia:List_of_Wikipedians_by_number_of_edits |enwiki's top 10,000]], excluding vanished-renames)
**non-ascii** (found [[ https://www.wikidata.org/wiki/Q13388884 | via ]]):
* 43: `Хмельницкий Константин`
* 46: `نبيل عبدالقادر عبدالوهاب`
* 50: `फ़ाराह् देसाईं खान`
* 52: `ศรีกฤษณะ รามจันทรา`
* 57: `ديفيد عادل وهبة خليل (حساب ملغي)`
**ascii** (hypothetical):
* 64: `Lorem ipsum dolor sit amet, consectetuer adipiscing elit. Aenean`
* 85: `Lorem ipsum dolor sit amet, consectetuer adipiscing elit. Aenean commodo ligula eget.`
* 255: `Lorem ipsum dolor sit amet, consectetuer adipiscing elit. Aenean commodo ligula eget dolor. Aenean massa. Cum sociis natoque penatibus et magnis dis parturient montes, nascetur ridiculus mus. Donec quam felis, ultricies nec, pellentesque eu, pretium quis,`
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Ancient/related tasks:
* {T4693} (Brion raised limit from 32 to 64, in 2005)
* {T4696} (reflects the confusingly named variable $wgMaxNameChars, which measures bytes not characters)
* {T25080}