The RAW files of my Nikon D800 are, even cropped to 5x4 ratio, 6144 x 4912 pixels. The lowest setting (¼ or 25%) brings the height down to 1228, not small enough for my 22' screen that has 1680 x 1050 pixels.
Only the 'fit to window' setting brings it down a bit more; on my screen to 20%, just enough for the height. I would like to be able to go to at least 75% of the 'fit to window' setting, but sometimes it's nice to see the image even smaller, say 400-600 pixel, so thumbnail-size but without the UI hassle surrounding the image in the contact sheet.
By the way, I never use that slider; only the Z, plus and minus keys.