How to find out the size in byte of a specific table

As most simple solution You can use smth like the following example from
http://www.oracle-base.com/DBA/DBAScript.asp?Monitoring\Show_Space.sql:
here's the code:
CREATE OR REPLACE
PROCEDURE show_space
( p_segname IN VARCHAR2,
  p_owner   IN VARCHAR2 DEFAULT user,
  p_type    IN VARCHAR2 DEFAULT 'TABLE' )
AS
  l_free_blks                 NUMBER;
  l_total_blocks              NUMBER;
  l_total_bytes               NUMBER;
  l_unused_blocks             NUMBER;
  l_unused_bytes              NUMBER;
  l_last_used_ext_file_id     NUMBER;
  l_last_used_ext_block_id        NUMBER;
  l_last_used_block           NUMBER;
 
  PROCEDURE p( p_label IN VARCHAR2, p_num IN NUMBER )
  IS
  BEGIN
     DBMS_OUTPUT.PUT_LINE( RPAD(p_label,40,'.') || p_num );
  END;
 
BEGIN
  DBMS_SPACE.FREE_BLOCKS (
    segment_owner     => p_owner,
    segment_name      => p_segname,
    segment_type      => p_type,
    freelist_group_id => 0,
    free_blks         => l_free_blks );

  DBMS_SPACE.UNUSED_SPACE (
    segment_owner             => p_owner,
    segment_name              => p_segname,
    segment_type              => p_type,
    total_blocks              => l_total_blocks,
    total_bytes               => l_total_bytes,
    unused_blocks             => l_unused_blocks,
    unused_bytes              => l_unused_bytes,
    last_used_extent_file_id  => l_last_used_ext_file_id,
    last_used_extent_block_id => l_last_used_ext_block_id,
    last_used_block           => l_last_used_block );
 
  p( 'Free Blocks', l_free_blks );
  p( 'Total Blocks', l_total_blocks );
  p( 'Total Bytes', l_total_bytes );
  p( 'Unused Blocks', l_unused_blocks );
  p( 'Unused Bytes', l_unused_bytes );
  p( 'Last Used Ext FileId', l_last_used_ext_file_id );
  p( 'Last Used Ext BlockId', l_last_used_ext_block_id );
  p( 'Last Used Block', l_LAST_USED_BLOCK );
END;
/




To try something for  a specific schema schema :
SELECT t.table_name AS "Table Name",
       t.num_rows AS "Rows",
       t.avg_row_len AS "Avg Row Len",
       Trunc((t.blocks * p.value)/1024) AS "Size KB",
       t.last_analyzed AS "Last Analyzed"
FROM   dba_tables t,
       v$parameter p
WHERE t.owner = Decode(Upper('&1'), 'ALL', t.owner, Upper('&1'))
AND   p.name = 'db_block_size'
ORDER by t.table_name; with &1 - Your schema name

Hope it helpfull,the mostly is the site,use it!!!

select * from  sys.dba_segments where segment_name = '<table name>' - there u can see space occupied by table - not filled with data, but allocated. u can select * from  sys.dba_segments where segment_name = '<index name of your table>' to see how much space are occupied by table's indexes
if u want to see free space of table - select * from dba_tables - if there are calculated statisitcs on table all data are there.

I had adopted this to handle simple partitions also, this looks like this:

PROCEDURE show_space
( p_segname IN VARCHAR2,
  p_owner   IN VARCHAR2 DEFAULT user,
  p_type    IN VARCHAR2 DEFAULT 'TABLE' )
AS
  l_free_blks                 NUMBER;
  l_total_blocks              NUMBER;
  l_total_bytes               NUMBER;
  l_unused_blocks             NUMBER;
  l_unused_bytes              NUMBER;
  l_last_used_ext_file_id     NUMBER;
  l_last_used_ext_block_id        NUMBER;
  l_last_used_block           NUMBER;
 
  PROCEDURE p( p_label IN VARCHAR2, p_num IN NUMBER )
  IS
  BEGIN
     DBMS_OUTPUT.PUT_LINE( RPAD(p_label,40,'.') || p_num );
  END;
 
BEGIN
FOR I IN (SELECT DISTINCT PARTITION_NAME FROM DBA_PART_COL_STATISTICS WHERE TABLE_NAME=p_segname)
LOOP
  DBMS_SPACE.FREE_BLOCKS (
    segment_owner     => p_owner,
    segment_name      => p_segname,
    segment_type      => p_type,
    freelist_group_id => 0,
    free_blks         => l_free_blks,-- ! PARTITIONS ONLY
    PARTITION_NAME =>I.PARTITION_NAME );
  DBMS_SPACE.UNUSED_SPACE (
    segment_owner             => p_owner,
    segment_name              => p_segname,
    segment_type              => p_type,
    total_blocks              => l_total_blocks,
    total_bytes               => l_total_bytes,
    unused_blocks             => l_unused_blocks,
    unused_bytes              => l_unused_bytes,
    last_used_extent_file_id  => l_last_used_ext_file_id,
    last_used_extent_block_id => l_last_used_ext_block_id,
    last_used_block           => l_last_used_block ,
    PARTITION_NAME =>I.PARTITION_NAME );
  p( 'For PARTITION NAME  '||I.PARTITION_NAME||' (KB)', l_total_bytes/1024 );
  p( 'Free Blocks', l_free_blks );
  p( 'Total Blocks', l_total_blocks );
  p( 'Total Bytes', l_total_bytes );
  p( 'Unused Blocks', l_unused_blocks );
  p( 'Unused Bytes', l_unused_bytes );
  p( 'Last Used Ext FileId', l_last_used_ext_file_id );
  p( 'Last Used Ext BlockId', l_last_used_ext_block_id );
  p( 'Last Used Block', l_LAST_USED_BLOCK );
END LOOP;
END;

The last one was for displaying the partition size in KBs

Konector gave you the simple answer to your first question:
"find out for a specific table it's size in bytes":

select bytes from sys.dba_segments where segment_name = '<table name>';

Or, if you are logged in as the schema owner:

select bytes from user_segments where segment_name = '<table name>';


What do you mean by "remaining size"?

Do you mean free space in allocated blocks for that table that have no data in them yet?

Do you mean free space in allocated blocks that had data, but now have space for more because some data has been deleted?

Or do you mean free space in the tablespace not yet allocated to this (or any) table, but available for this (or any) table?

In the procedure I supplied use p_segname  as a name for the particular table:

PROCEDURE show_space
( p_segname IN VARCHAR2,
  p_owner   IN VARCHAR2 DEFAULT user,
  p_type    IN VARCHAR2 DEFAULT 'TABLE' )