Unless things have changed recently, changing the format of a native field is not something that can be done.
If I were you, I would create a new UD field that has the proper length and change the UI to use that field instead.
Have you tried changing the format in Extended Properties? We’ve been able to make integers longer with this setting although I believe since this field is nvarchar(50) in the DB you may need to take a look at using another field.