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In Jira Cloud instances, Atlassian imposes a limit of 128KB or 131,072 characters 240 KiB or ≈ 245 KB for all custom field configurations. This limit is imposed to ensure performance and scalability of the cloud service.
This is the Atlassian documentation about Forge 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.