On-premises Instances
In on-premises instances (Data Center or Server), there is no strict limit on the number of characters allowed in custom field configurations. This means you can configure custom fields with a significant amount of 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.