[eluser]pickupman[/eluser]
You want to make sure that you are using the database for sessions. It's a little safer and allows a lot more data to be saved than the 4kb cookie option.
See [url="http://ellislab.com/codeigniter/user-guide/libraries/sessions.html"]Saving Session Data to a Database[/url], then you may have to adjust the field size for the user_data column. Read the [url="http://dev.mysql.com/doc/refman/5.0/en/storage-requirements.html"]MySQL storage requirements[/url] and find out what data type will hold your information. May LONGTEXT or MEDIUMTEXT.
Keep in mind oversizing you columns will also slow down your database, as well fragment it with a lot of read / writes for session information.