SCOTT:MYDB> select table_name,round((blocks*8),2)||'kb' "size"
  2  from dba_tables
  3  where table_name = 'MY_LARGE_TABLE';
MY_LARGE_TABLE      1231904kb

SCOTT:MYDB> select table_name,round((num_rows*avg_row_len/1024),2)||'kb' "size"
  2  from dba_tables
  3  where table_name = 'MY_LARGE_TABLE';
MY_LARGE_TABLE      438496.52kb

SCOTT:MYDB> select bytes/1024 from dba_segments
  2  where owner='SCOTT' and segment_name='MY_LARGE_TABLE';
   2059264