ORA-20000

This is a simple query that returns the size of the blob or clob.

select '&TABLE' as table_name, '&COLUMN_NAME' as column_name, sum(dba_segments.BYTES) / 1024 / 1024 as Mb
from   dba_segments
where  (dba_segments.owner, dba_segments.segment_name) in
       (
         -- lob segments
         select dba_lobs.owner, dba_lobs.segment_name
         from   dba_lobs
         where  dba_lobs.owner = '&OWNER' and
                dba_lobs.table_name = '&TABLE' and
                dba_lobs.column_name = '&COLUMN_NAME'
         union all
         -- lob index segments
         select dba_lobs.owner, dba_lobs.index_name
         from   dba_lobs
         where  dba_lobs.owner = '&OWNER' and
                dba_lobs.table_name = '&TABLE' and
                dba_lobs.column_name = '&COLUMN_NAME'
       );

See also:

Size of table in Oracle
Размер Clob