library/XenForo/DataWriter/User.php
---
Search for 'maxLength' => 50
'username'
=> array('type' => self::TYPE_STRING, 'required' => true, 'maxLength' => 50, 'verification' => array('$this', '_verifyUsername'), 'requiredError' => 'please_enter_valid_name'),
Change to 'maxLength' => 100
'username'
=> array('type' => self::TYPE_STRING, 'required' => true, 'maxLength' => 100, 'verification' => array('$this', '_verifyUsername'), 'requiredError' => 'please_enter_valid_name'),
---
Search for...