It looks like file size is the issue here when getting these [413] errors from the server: I’ve lowered the payload size progressively to the point where a ~200Kb base64 payload succeeds.
It would be helpful for HF to state an explicit limit, and whether the upload limit changes with a Pro account or when hosting the input URLs in Spaces/Hub v. external domains.
From this post it seems that others have problems uploading similarly ‘small’ files.