I thought the limit for MySQL was 500. Anyway if you modify the java.lang.String mySQL mapping in the standardjbosscmp-jdbc.xml file to VARCHAR(250), it should work.
I happened to encounter this today. I am also trying CMP with MySQL.
The solution is to change the type mapping from the default "Hypersonic SQL" to "mySQL."
My question now is, does anybody know how to use the MySQL AUTO_INCREMENT primary key feature with CMP?
According to the MySQL website, this can now be done with the new beta version of Connector/J. See http://www.mysql.com/products/connector-j/index.html.
Support for AUTO_INCREMENT was checked in and then commented out yesterday, because of a build ordering problem. Full support should be available in JBoss 4.0 later next week and it will be back ported to 3.2 after that.