ok, you are certainly use some plugins/theme - Leo???
if i run the query above on my test db - then the result looks:
f,e field ps_btmegamenu.content is of type text (max size = 65535B)
But i am using utf-8 - what means:
A text column can be up to 65,535 bytes.
An utf-8 character can be up to 3 bytes.
So... your actual limit can be 21,844 characters.
(see http://dev.mysql.com/doc/refman/5.1/en/string-type-overview.html)
therefore i suggest to increase the content field capacity to MEDIUMTEXT (length limit is 16M) or LONGTEXT (length limit is 4G.) at the tables that PS/or your plugins use