I just run a bunch of tests on my Nexus 5x (the screen size is the same as Nexus 5) using an image with a grid and I thought I should write down my findings here.
I have icons on two screens, so it is asymmetric: I have my main screen and a screen on the right, there is no screen on the left. The image has to be a little larger so that it scrolls nicely. The thing is that my phone shows _the middle_ of the image on the main screen. So the main screen is 1920×1080, when I scroll right 60 more pixels on the right become visible. Since there is nowhere to scroll left, 60 leftmost pixels of the image are never visible.
To sum up: one needs a 1920×1200 image (1200 = 60 + 1080 + 60). Leftmost 60 pixels are never visible.