What is the size limit for the internal historian storage?
There are some limits that you can apply for the internal historian provider: Tag History Providers - Ignition User Manual 8.1 - Ignition Documentation
If you’re on Standard Edition (with Tag Historian module), you can configure/adjust those limits. Just keep in mind that the internal SQLite-based historian is not going to be as performant as a full database solution.
If you’re on Edge, you’ve got 7 days or 10,000,000 points (non-configurable) as the limits for the Edge Historian internal history provider.