4x20 alphanumeric LCD with backlight. Backlight is efficient 4 .. 4.2 V drop (most often 22 or 27 Ohm resistor is used to limit backlight current from 5V).
4x20 is the only maintained geometry in lcd_display.c therefore the recommended size.
The blue LCD has somewhat smaller form-factor (narrow as the LCD itself: solder pads on the side) => fits into places where the green or red does not