+++++++++++++++++++++++++++++++
Minimum datafile size for locally managed tablespace with uniform extent size is 1 extent + 2 blocks + 1 byte. E.g. (suppose db_block_size = 8192):
+++++++++++++++++++++++++++++++
SQL> create tablespace tmp datafile '/tmp/tmp01.dbf' size 1064960 extent management local uniform size 1048576;
create tablespace tmp datafile '/tmp/tmp01.dbf' size 1064960 extent management local uniform size 1048576
*
ERROR at line 1:
ORA-03214: File Size specified is smaller than minimum required


SQL> create tablespace tmp datafile '/tmp/tmp01.dbf' size 1064961 extent management local uniform size 1048576;

Tablespace created.

1048576 = 8192*128
1064960 = 1048576+2*8192

SQL> drop tablespace tmp including contents and datafiles;

Tablespace dropped.

+++++++++++++++++++++++++++++++
Test with extent size 128k (131072):
+++++++++++++++++++++++++++++++
SQL> create tablespace tmp datafile '/tmp/tmp01.dbf' size 147456 extent management local uniform size 131072;
create tablespace tmp datafile '/tmp/tmp01.dbf' size 147456 extent management local uniform size 131072
*
ERROR at line 1:
ORA-03214: File Size specified is smaller than minimum required


SQL> create tablespace tmp datafile '/tmp/tmp01.dbf' size 147457 extent management local uniform size 131072;

Tablespace created.

SQL> select * from dba_free_space where tablespace_name = 'TMP';

TABLESPACE_NAME                   FILE_ID   BLOCK_ID      BYTES     BLOCKS
------------------------------ ---------- ---------- ---------- ----------
RELATIVE_FNO
------------
TMP                                     8          4     131072         16
           8


SQL> !ls -l /tmp/tmp01.dbf
-rw-r-----   1 oracle   dba       163840 Feb  4 10:18 /tmp/tmp01.dbf

+++++++++++++++++++++++++++++++
The OS level filesize is exactly 1 extent + 4 blocks (i.e. 163840 = 131072+4*8192).
+++++++++++++++++++++++++++++++




+++++++++++++++++++++++++++++++
When there's a segment in the tablespace, the datafile has to have extra 7 blocks + 1 byte (57345 bytes) to tightly compact data:
+++++++++++++++++++++++++++++++

131072 = 8192*16
450560 = 131072*3+8192*7

SQL> create tablespace tmp datafile '/tmp/tmp01.dbf' size 450560 extent management local uniform size 131072;

Tablespace created.

SQL> create table tmp (a number) tablespace tmp;

Table created.

SQL> alter table tmp allocate extent;

Table altered.

SQL> alter table tmp allocate extent;
alter table tmp allocate extent
*
ERROR at line 1:
ORA-01653: unable to extend table YONG.TMP by 16 in tablespace TMP


SQL> select * from dba_free_space where tablespace_name = 'TMP';

no rows selected

SQL> alter database datafile '/tmp/tmp01.dbf' resize 450561;

Database altered.

SQL> select * from dba_free_space where tablespace_name = 'TMP';

TABLESPACE_NAME                   FILE_ID   BLOCK_ID      BYTES     BLOCKS RELATIVE_FNO
------------------------------ ---------- ---------- ---------- ---------- ------------
TMP                                     8         41     131072         16            8

SQL> alter table tmp allocate extent;

Table altered.


+++++++++++++++++++++++++++++++
But if db_block_size is 16384, the tablespace needs space for the segment(s) plus 3 blocks + 1 byte (49153 bytes). So why 7 blocks + 1 byte for 8192 block size and 3 blocks + 1 byte for 16384? Because the datafile has to have 64k-1block+1 datafile header, i.e. 64*1024-8192+1=57345 and 64*1024-16384+1=49153 bytes, respectively. And this overhead doesn't count the OS level file overhead, which is another db_block_size.

Guideline to maximize datafile space usage:
if db_block_size=8192, create tablespace datafile size integer*extentsize+49153;
if db_block_size=16384, create tablespace datafile size integer*extentsize+57345;
+++++++++++++++++++++++++++++++

SQL> create tablespace tmp datafile '/tmp/tmp01.dbf' size 393216 reuse extent management local uniform size 131072;

Tablespace created.

SQL> create table tmp (a number) tablespace tmp;

Table created.

SQL> alter table tmp allocate extent;

Table altered.

SQL> alter table tmp allocate extent;
alter table tmp allocate extent
*
ERROR at line 1:
ORA-01653: unable to extend table SYSTEM.TMP by 8 in tablespace TMP


SQL> select * from dba_free_space where tablespace_name = 'TMP';

no rows selected

SQL> alter database datafile '/tmp/tmp01.dbf' resize 442368;

Database altered.

SQL> select * from dba_free_space where tablespace_name = 'TMP';

no rows selected

SQL> alter database datafile '/tmp/tmp01.dbf' resize 442369;

Database altered.

SQL> select * from dba_free_space where tablespace_name = 'TMP';

TABLESPACE_NAME                   FILE_ID   BLOCK_ID      BYTES     BLOCKS RELATIVE_FNO
------------------------------ ---------- ---------- ---------- ---------- ------------
TMP                                    15         21     131072          8           15

SQL> alter table tmp allocate extent;

Table altered.