On-premises Instances
In On-Premises instances, there is no strict limit on the number of characters allowed in custom field configurations. This means you can configure custom fields with significant data without worrying about platform-imposed limits.
Cloud Instances
In Jira Cloud instances, Atlassian imposes a limit of 128KB or 131,072 characters for all custom field configurations. This limit is imposed to ensure performance and scalability of the cloud service.