Phone wallpaper sizes look like a list of unrelated numbers: 1080×2340, 1170×2532, 1080×2400, 1440×3120. Divide each pair and most of them land close together, around 19.5:9 or 20:9, with the screen a little more than twice as tall as it is wide. That ratio matters more than the pixel count, because it decides how much of the picture gets cut off.
A 1440×3120 file is a comfortable place to start. On a 1080×2340 phone it scales down by exactly a quarter and nothing is lost. On a 1080×2400 phone it is scaled down slightly less to reach the extra height, and about 14 pixels go from each side, which nobody will miss. A landscape 4K photo does far worse: its 2160 pixels of height fall short of 2340, so the phone enlarges it a little and keeps a slice about 997 pixels wide, roughly a quarter of the frame.
There are two exceptions worth knowing. The inner screen of a foldable is nearly square, around 2160×1856, so the same tall file is enlarged 1.5 times and shows less than 40 percent of its height. Lock screens with a depth or parallax effect also zoom in slightly so the image has room to shift, which is a good reason to keep the subject away from the edges. Once the shape matches and the file is at least as tall as the display, the remaining decision is where the subject sits after the clock has taken the upper part of the glass.
