There's a limit of 32K bytes on the length of string literals - may be this is your problem. You can try to work around this limitation by concatenating together several literal strings, each of them being shorter than 32K. However, you'll hit the 64K limit on the length of a SQL statement.
A proper way to work with LOBs depends on the programming language you use. In java, you would use PreparedStatement.setCharacterStream()