List all tables of a given user in Oracle

This will get all the tables where the "JOHN" user is the owner:

SELECT * FROM USER_TABLES;

or

SELECT * FROM ALL_TABLES WHERE OWNER = 'JOHN';

([TL;DR] 'JOHN' typically needs to be in upper-case. Assuming that the user john was created using the CREATE USER john ... statement then Oracle's default behaviour is to convert all object names (i.e. tables, columns, users, etc) to upper case. When you query the data-dictionary the table details will be stored in this case (and not the case you used in the original command unless you wrap it in double quotes).)

To list the table you can use

SELECT * FROM ALL_TABLES WHERE OWNER = 'JOHN';

TO see the size of the schema you can use

SELECT sum(bytes)
  FROM dba_segments
 WHERE owner = 'JOHN'

Since you are logged in as the schema owner, you can also use

SELECT SUM(bytes)
  FROM user_segments