I looked in my MySQL database and the field 'summary' in the table 'course_sections' is of type 'LONGTEXT' which according to http://dev.mysql.com/doc/refman/5.0/en/storage-requirements.html gives you one under 4 billion characters.
So, all you need is around one million monkies typing several words each and you'll have your limit reached soon ;)
Cheers,
Gareth