dba_segments is used to get schema size in Oracle. Schema is simply a user inside the database. To find growth of schema, you can check schema size. schema owner represents the Oracle user that owns all your database objects. So to check the size of the schema means to get all the database objects owned by that user.
Query to check owner size in Oracle:
SELECT sum(bytes)/1024/1024/1024 as "Size in GB"
from dba_segments
WHERE owner = UPPER('&schema_name');
To get all schema size from Oracle database:
select owner,sum(bytes)/1024/1024/1024 as "Size in GB" from dba_segments group by owner;