JRA-38732 which mentions a setting jira.text.field.character.limit to limit the size of the Description and Environment fields, and also Comments.
https://confluence.atlassian.com/display/JIRA/JIRA+5.0.3+Release+Notes says "This character limit is specified through the jira.text.field.character.limit property, which can be accessed through the Advanced Settings page in the JIRA administration area. Specifying a value of 0 (i.e. zero, which is the default value) for this property disables this feature."
Request: please document this setting in https://confluence.atlassian.com/display/JIRA/Configuring+Advanced+Settings (and update the list of other available settings as well)