In designing background wallpapers for all the potential screen resolutions that Apple users might encounter, I referenced Apple’s site to find the pixel dimensions of the iPhone 6+ display. What I found on their site was very misleading. If you want to cut together your own custom wallpaper that looks good on the screen in portrait or landscape orientation, it is practically impossible to find the answer at Apple.com.
John Carey of Fifty Foot Shadows has figured out what the ideal pixel dimensions are for custom wallpapers: 2706x2706. This will give you the results you are probably looking for when you turn your 6+ to a landscape orientation while viewing your lock screen or home screen.
As a heads up, there are a great number of custom wallpapers online that are not this ideal size, so they will not appear crisply on your iPhone. I need to update the Focus Collection. Who knows if and when other designers will catch on and update their stock. ∞