By default super title is stored in database with the maximum of 150 characters. You need to be sure that you have set the proper encoding for your forums, so each 1 symbol stored takes 1 byte in database. When the improper encoding is set, it could happen that some of the specific characters will be saved in special HTML symbols format. For example, russian symbol 'a' if the encoding iso-8859-1 is set, will be saved as & #1072; and that way will take 7 bytes in database.
If you would like to set larger amount of characters for that field, you will need to set it up in database as 'text' type. With the following command for example:
alter table minibbtable_forums change forum_group forum_group text not null default '';
Then also modify the following templates:
where instead of
specify TEXTAREA for this field, like