I had the same issue when trying to load a similar size texture and concluded that it was indeed reaching Terragen's limit. I couldn't get anything to load above 56k as a single image.
What you can do as a workaround is split your 60k image in two squares, and manually put their coordinate in a geog image shader. I succesfully loaded the equivalent of a 86k image file that way.
Regarding the displacement, increasing the size from 43k to 60k won't change a thing except using more memory. You will not create extra details, just blur the existing one by scaling the map up.