2880 x 2560 ?
That will make it fill the screen?
Screen resolution of the Droid Turbo is 1440 X 2560. By setting the width of the wallpaper to twice the size of width of the screen, when you swipe left/right from the home page screen to adjacent screens there is additional "real estate" so you don't run out of background image. 2880X2560 is the size of the official Moto wallpapers that come with the Turbo.
You can search for larger images as well, then use a photo editing program to resize them to 2880X2560 with a resolution of 72 px/inch.