Use dbms_lob.getlength to get the length of a clob data : DBMS_LOB « System Packages « Oracle PL/SQL Tutorial

SQL>
SQL> create or replace procedure print_clob( p_clob in clob )
  2  as
  3       l_offset number default 1;
  4  begin
  5     loop
  6       exit when l_offset > dbms_lob.getlength(p_clob);
  7       dbms_output.put_line( dbms_lob.substr( p_clob, 255, l_offset ) );
  8       l_offset := l_offset + 255;
  9     end loop;
 10  end;
 11  /

Procedure created.

SQL>
SQL>

31.12.DBMS_LOB
	31.12.1.	DBMS_LOB Package
	31.12.2.	Print clob data out
	31.12.3.	Use dbms_lob.getlength to get the length of a clob data
	31.12.4.	Show Java source file
	31.12.5.	Use dbms_lob package to deal with clob data
	31.12.6.	Use dbms_lob.getlength and dbms_lob.substr with blob type column
	31.12.7.	Use dbms_lob.substr to get part of clob data
	31.12.8.	DBMS_LOB.READ
	31.12.9.	Load a file