How to find object/schema size

Find SCHEMA SIZE or USER SIZE

SELECT tablespace_name,
Sum(bytes)/1024/1024 AS total_size_mb
FROM dba_segments
WHERE owner = Upper('&User_Name')
GROUP BY owner, rollup(tablespace_name)

Find OBJECT Size

select segment_name, sum(bytes/(1024*1024)) size_mb from user_segments
where segment_type = 'TABLE' and segment_name='&SEGMENT_NAME'
group by segment_name