How to find size of LOB

Select b.table_name,b.Column_name,c.data_type,a.Segment_name,a."size"
from
(Select Segment_name , (bytes/(1024*1024*1024)) "size"
from User_Segments
where (bytes/(1024*1024*1024))>0.5 )a,
(Select Table_name,Column_name,Segment_name
from User_Lobs)b,
(Select table_name,Column_Name,Data_type from User_Tab_Columns
Where Data_Type in ('CLOB','BLOB','LONG','LONG RAW') ) c
Where a.segment_name=b.segment_name
and b.table_name=c.table_name
and b.column_name=c.column_name
Order by c.data_type
/