MEDIUMTEXT [CHARACTER SET charset_name] [COLLATE collation_name]

A TEXT column with a maximum length of 16,777,215 (224 − 1)
characters. The effective maximum length is less if the value contains
multibyte characters. Each MEDIUMTEXT value is stored using a 3-byte
length prefix that indicates the number of bytes in the value.

URL: http://dev.mysql.com/doc/refman/5.6/en/string-type-overview.html

