create table lobs_tab (
    id number,
    clob_col clob
);

insert into lobs_tab (id, clob_col) values (1, empty_clob());

select bytes from user_segments where segment_name = 'SYS_LOB0000033146C00002$$';

 
BYTES
--------
131072
 
update lobs_tab set clob_col = rpad(clob_col, 1000000, '!');
select bytes from user_segments where segment_name = 'SYS_LOB0000033146C00002$$';

 
BYTES
----------
3342336
 
select dbms_lob.getlength(clob_col) len from lobs_tab;

 
LEN
----------
1000000
 

update lobs_tab set clob_col = 'test of update' || clob_col where id = 1;
select bytes from user_segments where segment_name = 'SYS_LOB0000033146C00002$$';

 
BYTES
----------
5439488
 
select dbms_lob.getlength(clob_col) len from lobs_tab;

 LEN
----------
1000014
 

  CREATE TABLE "TEST_SCHEMA"."LOBS_TAB" 
   (	"ID" NUMBER(5,0), 
	"CLOB_COL" CLOB
   ) SEGMENT CREATION IMMEDIATE 
  PCTFREE 10 PCTUSED 40 INITRANS 1 MAXTRANS 255 
 NOCOMPRESS LOGGING
  STORAGE(INITIAL 65536 NEXT 1048576 MINEXTENTS 1 MAXEXTENTS 2147483645
  PCTINCREASE 0 FREELISTS 1 FREELIST GROUPS 1
  BUFFER_POOL DEFAULT FLASH_CACHE DEFAULT CELL_FLASH_CACHE DEFAULT)
  TABLESPACE "TEST_SCHEMA" 
 LOB ("CLOB_COL") STORE AS SECUREFILE (
  TABLESPACE "TEST_SCHEMA" ENABLE STORAGE IN ROW CHUNK 8192
  NOCACHE LOGGING  NOCOMPRESS  KEEP_DUPLICATES 
  STORAGE(INITIAL 106496 NEXT 1048576 MINEXTENTS 1 MAXEXTENTS 2147483645
  PCTINCREASE 0
  BUFFER_POOL DEFAULT FLASH_CACHE DEFAULT CELL_FLASH_CACHE DEFAULT)) ;