First, do you really want an 8-bit heightmap of the entire world? If you reduce it to only 256 levels, there won't be enough detail left in it to make it worthwhile.
And do you really want a normal map (the purple image at the bottom of the screen)? Or do you want to stop at the point where you have the heightmap? The tutorial's Mac-only instructions start right after the heightmap's been prepared. If you continue, you'll wind up with the normals. If you instead stop in step 3 and go to Image > Mode > 8 bits/Channel and Image > Mode > Grayscale, you should have what you want. If you're not running the script to make the normals, you don't necessarily need to resize, either. Seems to me that doing so misses the point of getting this enormous file to begin with—why download such a huge image if you're just going to throw away 96% of the pixels?