Session userdata unsetting if too many variables are set...? Help Please

[eluser]qwang[/eluser]
[quote author="castor007" date="1283282075"]If you use MSSQL and a TEXT field for user_data don't forget to change PHP.INI settings.

default length in php.ini for a TEXT field is 4096 and not 8192 or more[/quote]

i found this property is closed default, but i found the information in the phpinfo() output
so where php get the default value?
thank you