dal manuale:

http://dev.mysql.com/doc/refman/5.0/...-overview.html

TEXT[(M)] [CHARACTER SET charset_name] [COLLATE collation_name]

A TEXT column with a maximum length of 65,535 (216 – 1) characters. The effective maximum length is less if the value contains multi-byte characters. Each TEXT value is stored using a two-byte length prefix that indicates the number of bytes in the value.

An optional length M can be given for this type. If this is done, MySQL creates the column as the smallest TEXT type large enough to hold values M characters long.
usa mediumtext tiene 16 mb