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The help section has a special page on limits of various parameters and attributes for Target. Entity.ids are limited to 1000 characters. However, in practice it is more convenient to keep the entity.id's to around 50 characters or less because they are easier to view in the UI (not truncated) and easier to work with in the Recommendations data CSV downloads for an activity that you might use for troubleshooting.
The help section has a special page on limits of various parameters and attributes for Target. Entity.ids are limited to 1000 characters. However, in practice it is more convenient to keep the entity.id's to around 50 characters or less because they are easier to view in the UI (not truncated) and easier to work with in the Recommendations data CSV downloads for an activity that you might use for troubleshooting.