"Can I change the Length of column in OOTB table?"
Yes, you can!
You can alter OOTB table properties.
Second, to use email address as login property, then either from UI or customizing the ProfileFormhandler make the login=emailAddress then rest will be taken care.
Thanks for reply. Client want to keep the flexibility to keep login Id (for old user) Or email Id. (new users) as loginId.