Think about your favourite way to view a collection of images (just to enjoy the images, not for pixel-peeping or special scrutiny).
What is your usual preferred viewing distance relative to the length of the image diagonal?
For myself, I usually view images on a 27" iMac. Most of my images are 4:3 aspect ratio. I'll ignore the others that can be any ratio from 1:1 to 4:1 or even more. I generally view the screen from about 22" away, which is approximately the length of the diagonal of a 4:3 image that fits the height of the screen.
How do you like to view images just to enjoy the images?