The only place I know it to be limited is in the mySQL database itself. I believe the field that holds the post_text is a text field which means it is limited to 65535 characters. Now that would be one hell of a long news post. But even if you enter too much data into a field it should just truncate the data, not die altogether?
Anyways you could dig into the tables and try changing that field to Longtext which is capable of holding 4294967295 characters. Although not sure how this would change the performance of the db as a whole.
For more info see
the mySql site
Make sure you have your db backed up before making any changes. Also I am not 100% sure the problem is with the db in this case just talking out loud here
