Oh gee, that's SUCH a good question, and one to which there's no precise answer, I'm afraid.
These will be whatever your camera took. How many megapixels is your camera?
I reckon Facebook size (2048px on the long edge) would be fine for this.
I had just been trying it too: https://max.mailbigfile.com/c87c6df4269bbe6f610f4550a555ea4b/listFiles.php
Why are you using the dark version of the photo again?