ORA -12953: The request exceed the maximum allowed database size of 11GB

What should be done to add or resize the datafile?

Nothing can be done. The XE version is limited to 11GB of data, see this link

Oracle Database Express Edition
Oracle Database Express Edition (Oracle Database XE) is an entry-level edition of Oracle Database that is quick to download, simple to install and manage, and is free to develop, deploy, and distribute. Oracle Database XE makes it easy to upgrade to the other editions of Oracle without costly and complex migrations. Oracle Database XE can be installed on any size machine with any number of CPUs, stores up to 11 GB of user data, using up to 1 GB of memory, and using only one CPU on the host machine. Support is provided by an online forum.

You can purchase Standard or Enterprise edition, which don't have this restriction (but you must pay for it).

You can also register in Oracle's Technology Network and use any of Oracle's product (including Oracle's Standard and Enterprice database editions from their download site) for free, but only for testing and development purposes.

See this licence for details:

License Rights and Restrictions
Oracle grants You a nonexclusive, nontransferable, limited license to internally use the Programs, subject to the restrictions stated in this Agreement, only for the purpose of developing, testing, prototyping, and demonstrating Your application and only as long as Your application has not been used for any data processing, business, commercial, or production purposes, and not for any other purpose.

As krokodilko says, it's a license limit.

Try to find some garbage or old data to delete... maybe some log table or something like that!

select owner||'.'||segment_name,segment_type, trunc(sum(bytes/1024/1024/1024),5) GB from dba_segmentsgroup by owner,segment_name,segment_typeorder by 3 desc;