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. Defaults to 65 if not specified or less than 0.
commitmessage.maxLineLength : Maximum length of a line in the commit message's body. Defaults to 70 if not specified or less than 0.
commitmessage.longLinesThreshold : Percentage of commit message lines allowed to exceed the maximum length before a warning or error is generated. Defaults to 33 if not specified or less than 0.
commitmessage.rejectTooLong : If set to true, reject commits whose subject or line length exceeds the maximum allowed length. If not specified, defaults to false.