It's actually 2662x2662 which is the resolution of the Apple official wallpapers.
Using a lower resolution results in a less than satisfying result as dekoapp.com rightly says:
"iOS is of course happy to use smaller or bigger files and resize or crop them to fit, but this can result in artefacts as the images get interpolated. So for pixel-perfection use the dimensions below or grab the Photoshop templates."
This, and Apple hasn't made the Landscape wallpaper look nice on non official wallpapers for 6 Plus...