You can download a 2000 km x 800 km region, but only with a "Coarse" resolution and not a "Precise" resolution.
With a "Coarse" resolution, the size of the downloaded file will be something like 22,000 x 8,000 pixels so you will need Instant Terra Unlimited or Instant Terra Pro to be able to import it, as Instant Terra is limited to 8,192 x 8,192 terrains.
The maximum size that can be downloaded when resolution = "Precise" seems to be 450,000 km², which is below the size of your region (1,600,000 km²).
(And again, when the download fails, it's necessary to close and restart Instant Terra to be able to download other regions).
We will disable the "Precise" resolution when the region to download is too large.
Thanks a lot for your feedback.