How to change initial extent of an existing partition
Even if its an empty partition, MOVE is the way to do it:

create table t1 (c1 number, c2 varchar2(100))
segment creation immediate
partition by range (c1)
(
  partition p1 values less than (10),
  partition p2 values less than (20)
);

SQL&gt; select partition_name, initial_extent, next_extent
     from user_tab_partitions where table_name = 'T1';

PARTITION_NAME       INITIAL_EXTENT NEXT_EXTENT
-------------------- -------------- -----------
P1                          8388608     1048576
P2                          8388608     1048576


Default 8M/1M. Now move the partition:

SQL&gt; alter table t1 move partition p1 storage (initial 65536 next 65536);

Table altered.

SQL&gt; select partition_name, initial_extent, next_extent from
     user_tab_partitions where table_name = 'T1';

PARTITION_NAME       INITIAL_EXTENT NEXT_EXTENT
-------------------- -------------- -----------
P1                            65536       65536
P2                          8388608     1048576


Modifying default attributes will have its effect only on partitions created after setting it:

SQL&gt; alter table t1 modify default attributes storage ( initial 1M next 1M );

Table altered.

SQL&gt; alter table t1 add partition p3 values less than (30);

Table altered.

SQL&gt; select partition_name, initial_extent, next_extent from
     user_tab_partitions where table_name = 'T1';

PARTITION_NAME       INITIAL_EXTENT NEXT_EXTENT
-------------------- -------------- -----------
P1                            65536       65536
P2                          8388608     1048576
P3                          1048576     1048576