If your code contains the characters
]]> then you'll find that it doesn't - WordPress modifies this itself.
WordPress stores the custom field contents in a MySQL table using the
longtext format. This can hold over 4 billion characters.
It has been tested and been found valid from PHP 4 upwards.
Please note, however, that the minimum for WordPress is now PHP 5.2.4. Even though this plugin supports a lower version, I am not coding specifically to achieve this - therefore this minimum may change in the future.