1920x1920 would mean that your phone would be full screen, pixel by pixel in both landscape and portrait but it would not scroll. I cant speak for all launchers but with Nova, anything over 1080 width (in portrait) will be divided by how many screens in that direction for scrolling.
So for example, if I have a really wide picture, but only 3 screens, the scrolling effect of the wallpaper will be a large difference. The more screens I add, the smaller that difference will become.
For me, I want a big choice of wallpapers without editing so, I go for as close a match to the 1920 height that I can get without going under.. so on interfacelift I go by 16:9, 3840x2160 - sure I know I'm 140 over on the height... If I could be bothered, I'd bulk trim them