<<rows per block>> =
  floor( <<database block size>> *
          (1 - <<pctfree of table>>/100) /
          <<size of row>> ) + 1

<<size of table>> =
  ceil( <<number of rows in table>> / <<rows per block>> ) *
    <<database block size>>

SQL> create table foo(
  2    foo_id number,
  3    foo_str varchar2(100)
  4  );

Table created.

<<rows per block>> =
  floor( 8192 *
          (1 - 10/100) /
          56 ) + 1

<<size of table>> =
  ceil( 1000000 / 132 ) *
    8192

SQL> ed
Wrote file afiedt.buf

  1  insert into foo
  2    select level, dbms_random.string( 'p', dbms_random.value(1,100))
  3      from dual
  4*  connect by level <= 1000000
SQL> /

1000000 rows created.

SQL> exec dbms_stats.gather_table_stats( 'SCOTT', 'FOO' );

PL/SQL procedure successfully completed.
SQL> ed
Wrote file afiedt.buf

  1  select avg_row_len, num_rows
  2    from user_tables
  3*  where table_name = 'FOO'
SQL> /

AVG_ROW_LEN   NUM_ROWS
----------- ----------
         56    1000000

SQL> select sum(bytes)/1024/1024 mb
  2    from user_segments
  3   where segment_name = 'FOO';

        MB
----------
        72

SQL> ed
Wrote file afiedt.buf

  1  select bytes, count(*)
  2    from user_extents
  3   where segment_name = 'FOO'
  4   group by bytes
  5*  order by bytes
SQL> /

     BYTES   COUNT(*)
---------- ----------
     65536         16
   1048576         63
   8388608          1

SQL> ed
Wrote file afiedt.buf

  1  DECLARE
  2   l_space_used NUMBER;
  3   l_space_allocated NUMBER;
  4   l_chained_pct NUMBER;
  5  BEGIN
  6    dbms_space.object_space_usage(
  7      'SCOTT',
  8      'FOO',
  9      'TABLE',
 10      NULL,
 11      l_space_used,
 12      l_space_allocated,
 13      l_chained_pct);
 14    dbms_output.put_line('Space Used: ' || TO_CHAR(round(l_space_used/1024/1024,2)) || ' MB');
 15    dbms_output.put_line('Space Allocated: ' || TO_CHAR(l_space_allocated/1024/1024) || ' MB');
 16    dbms_output.put_line('Chained Percentage: ' || TO_CHAR(l_chained_pct));
 17* END;
SQL> /
Space Used: 66.22 MB
Space Allocated: 72 MB
Chained Percentage: 0

PL/SQL procedure successfully completed.

SQL> ed
Wrote file afiedt.buf

  1  DECLARE
  2   l_used_bytes NUMBER;
  3   l_allocated_bytes NUMBER;
  4  BEGIN
  5    dbms_space.create_table_cost('USERS',
  6                                 56,
  7                                 1000000,
  8                                 10,
  9                                 l_used_bytes,
 10                                 l_allocated_bytes);
 11    dbms_output.put_line('Used Bytes: ' || TO_CHAR(round(l_used_bytes/1024/1024,2)) || ' MB');
 12    dbms_output.put_line('Alloc Bytes: ' || TO_CHAR(l_allocated_bytes/1024/1024) || ' MB');
 13* END;
SQL> /
Used Bytes: 63.52 MB
Alloc Bytes: 64 MB

PL/SQL procedure successfully completed.

SQL> ed
Wrote file afiedt.buf

  1  DECLARE
  2   l_used_bytes NUMBER;
  3   l_allocated_bytes NUMBER;
  4   l_cols sys.create_table_cost_columns;
  5  BEGIN
  6    l_cols := sys.create_table_cost_columns(
  7            sys.create_table_cost_colinfo('NUMBER',10),
  8            sys.create_table_cost_colinfo('VARCHAR2',100));
  9    dbms_space.create_table_cost('USERS',
 10                                 l_cols,
 11                                 1000000,
 12                                 10,
 13                                 l_used_bytes,
 14                                 l_allocated_bytes);
 15    dbms_output.put_line('Used Bytes: ' || TO_CHAR(round(l_used_bytes/1024/1024,2)) || ' MB');
 16    dbms_output.put_line('Alloc Bytes: ' || TO_CHAR(l_allocated_bytes/1024/1024) || ' MB');
 17* END;
SQL> /
Used Bytes: 67.94 MB
Alloc Bytes: 72 MB

PL/SQL procedure successfully completed.