Try to go to the options and set 100 as max username length, then try to edit a user in the admin panel, it will complain that the max usernane lenght is 50.
That is correct - the max length is always 50 characters based on the column length. Any changes would require modification to the DB and files. (I have added a note mentioning this to the option.)
I'm guessing it to be the value in XenForo_DataWriter_User.
Code:
'username' => array( 'maxLength' => 50)
My question is simply why not to use a variable there instead of the hardcoded value? Just specific in the ACP that values longer than 50 requires a database modification...