On-premises Instances
In onOn-premises instances (Data Center or Server)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 a significant amount of data without worrying about platform-imposed limits.
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In Jira Cloud instances, Atlassian imposes a limit of 128KB or 131,072 characters for all 240 KiB or ≈ 245 KB for custom field configurations. This limit is imposed to ensure performance and scalability of the cloud service.
This is the Atlassian documentation about Forge hosted storage key and object where you can find this information (under the paragraph “Forge hosted storage key and object size limits”): https://developer.atlassian.com/platform/forge/platform-quotas-and-limits/#forge-hosted-storage-key-and-object-size-limits.