This is an extract from Oracle documentation -
You can use table sizes to estimate the maximum space needed. Table sizes can be found in the USER_SEGMENTS view of the Oracle data dictionary. The following query displays disk usage for all tables:
select sum(bytes) from user_segments where segment_type='TABLE';
The result of the query does not include disk space used for data stored in LOB (large object) or VARRAY columns or partitions.
Santosh Jadhav
8i OCP DBA