Oracle schema size  ... 

DEFINE schema_name = 'XXXX'
set linesize 300

col schema_size_gb  for 9999999.99

SELECT sum(sizegb) schema_size_gb FROM (
-- tablesize !!!
--SELECT *  FROM (
SELECT
owner, object_name, object_type, table_name, ROUND(bytes)/1024/1024/1024 AS sizegb,tablespace_name, extents, initial_extent,
ROUND(Sum(bytes/1024/1024) OVER (PARTITION BY table_name)) AS total_table_MB
FROM (
-- Tables
SELECT owner, segment_name AS object_name, 'TABLE' AS object_type,segment_name AS table_name, bytes,tablespace_name, extents, initial_extent
FROM dba_segments
WHERE segment_type IN ('TABLE', 'TABLE PARTITION', 'TABLE SUBPARTITION')
UNION ALL
-- Indexes
SELECT i.owner, i.index_name AS object_name, 'INDEX' AS object_type,i.table_name, s.bytes,s.tablespace_name, s.extents, s.initial_extent
FROM dba_indexes i, dba_segments s
WHERE s.segment_name = i.index_name
AND s.owner = i.owner
AND s.segment_type IN ('INDEX', 'INDEX PARTITION', 'INDEX SUBPARTITION')
--LOB Segments
UNION ALL
SELECT l.owner, l.column_name AS object_name, 'LOB_COLUMN' AS object_type,l.table_name, s.bytes,s.tablespace_name, s.extents, s.initial_extent
FROM dba_lobs l, dba_segments s
WHERE s.segment_name = l.segment_name
AND s.owner = l.owner
AND s.segment_type = 'LOBSEGMENT'
-- LOB Indexes
UNION ALL
SELECT l.owner, l.column_name AS object_name, 'LOB_INDEX' AS object_type,l.table_name, s.bytes,s.tablespace_name, s.extents, s.initial_extent
FROM dba_lobs l, dba_segments s
WHERE s.segment_name = l.index_name
AND s.owner = l.owner
AND s.segment_type = 'LOBINDEX'
)
WHERE owner in UPPER('&schema_name')
)
--WHERE total_table_MB > 10
--ORDER BY total_table_MB DESC, MB DESC
/

====

with count 

set linesize 100 pagesize 200 
col Owner for a28
select obj.owner "Owner", obj_cnt "Objects",decode(seg_size, NULL, 0, seg_size) "size GB"
from ( select owner, count(*) obj_cnt from dba_objects group by owner) obj,
     ( select owner, ceil(sum(bytes)/1024/1024/1024) seg_size from dba_segments group by owner) segment
where obj.owner = segment.owner(+)
order by 3 desc, 2 desc, 1;