The maximum lengths of the subject and message body can be configured in the standard Gerrit config file gerrit.config.
commitmessage.maxSubjectLength : Maximum length of the commit message's subject line. If not specified, defaults to 65.
commitmessage.maxLineLength : Maximum length of a line in the commit message's body. If not specified, defaults to 70.
commitmessage.rejectTooLong : If set to true, reject commits whose subject or line length exceeds the maximum allowed length. If not specified, defaults to false.