How to check lob segments size

set linesize 300
col table_name format a20;
col column_name format a15;
col segment_name format a40;
col bytes format 999;
col TABLESPACE_NAME format a20;
select table_name, column_name,a.bytes/1024/1024/1024 as gigas, segment_name,a.TABLESPACE_NAME
from dba_segments a join dba_lobs b
using (owner, segment_name)
where b.table_name = 'table_name';