As the question says, I want to open a gallery in slideshow mode with all the big pictures scaled down to fit the screen and all the small ones scaled up to fit the screen as well. How can I do it?
Maybe you can combine the --scale-down and the --auto-zoom flags?
feh -Z. /path/to/images
-., --scale-down - automatically scale down images to fit screen size
-Z, --auto-zoom - zoom picture to screen size in fullscreen/geom mode