It's actually 852 x 1608 for iPhone 6 and 2662 x 2662 for iPhone 6 Plus. (Like the iPad, 6 Plus' home screen rotates to landscape, so the wallpaper should be long in both dimensions.)
http://dekoapp.com/parallax/
I just edited a bunch of super hires wallpapers ( 4096x3072) to fit this size and they still appear slightly fuzzy on my iPhone 6 Plus. While Apple's images still seem sharp and crisp. So I'm still not sure we've hit the proper resolution.
EDIT:
I also read this and tried it....
"I want to get the record straight on the 6 Plus. The screen resolution is 1920 by 1080, but it sounds like the *optimal* resolution is 2208 by 1242. It sounds like what happens if you have an image at 1920 by 1080 is that the software blows it up to 2208 by 1242 to render it and then shrinks it back down to 1920 by 1080 to display it. So, should we all by looking for wallpaper images at resolution 2208 by 1242?"
I made a same wallpaper using the same image and using this resolution it was slightly less blurry and more crisp. But this resolution doesn't take into account the landscape mode. So when you flip it the image zooms in where Apple's actually just rotates.
iPhone 6 - Parallax: 1592-by-893-pixel