User accounts, authentication: $wgMaxNameChars
Maximum number of bytes in username.
Introduced in version:1.2.0
Removed in version:still in use
Allowed values:(integer)
Default value:255 (1.5+)
32 (1.2-1.4)

Details edit

Sets the maximum number of bytes in username.

All existing usernames on a wiki consisting of more bytes will become invalid after you set this parameter to a lower value.

User::isUsableName() (and thus the "Create account" form) enforces a lower limit of 235 bytes, to avoid unreachable subpages such as User_talk:Username/monobook.css (task T25080).