The 5.8" display is bigger than the 5.5" Plus display, just it has a smaller ergonomic footprint with less bezels.
The X often functions with similar screen size to a 4.7” iPhone. It is goofy being so tall and thin. The Plus give you 5.5” of useable space at all times with no notch in the way blocking content. The wider screen also makes it easier to type in portrait. In everyday use the Plus is still the champ of screen space.