The ISO is 800 MB, but Rufus failed. Solution: You likely downloaded a fake "Hiren 15.6" that is actually the PE version or a virus. Delete it and re-download from the sources above (the real file is ~565 MB).
There is no official "Hiren’s BootCD 15.6." Download Hiren Boot 15.6 Iso
Hiren’s BootCD 15.2 (often referred to as 15.6 due to common community mislabeling; the final legacy version was technically 15.2, but many archives list it as 15.6) was the culminating release of the original "DOS + Mini XP" era. The ISO is 800 MB, but Rufus failed