This option's value should correspond to the size of the 'post_text' field under 'posts' table. By default this field has a 'TEXT' string type, and regarding mySQL manual
, it's allowed to store up to 65536 bytes in such field. That's why putting 900000 will not give a big effect, since mySQL will cut spare chars anyway.
For using bigger values, you should change the type of the above mentioned field to MEDIUMTEXT or LONGTEXT.
"Very large" documents are not actually suitable for searched context. As I heard, search engines most likely could even ignore text documents larger than 100 Kb in size. It of course depends on the nature of your forums, but from my experience, having TEXT field and $post_text_maxlength set to 30000 is enough for almost all kind of even big texts.