I was also looking for a solution to this, and like you, I couldn't get adjust-open to work either. I also couldn't find any discussion about this in the Zathura issue tracker. The other answer mentioning set adjust_window width doesn't work.
Unfortunately, I couldn't find a solution within Zathura itself, but after some fiddling with xdotool, I whipped up the following shell script:
#!/bin/sh
zathura "$@" & PID="$!"
while true; do
window_id="$(xdotool search --onlyvisible --pid "$PID")"
if [ -n "$window_id" ]; then
xdotool windowactivate --sync "$window_id" windowfocus --sync "$window_id" \
key s key --delay 0 g g
break
fi
done
This launches zathura and waits for a window to be created for that process ID. Then it activates and focuses the window, and sends the s keystroke to fit the document to the window width, followed by gg to go to the beginning. I don't like Zathura remembering the document position, but you can remove the second key --delay 0 g g if you don't need this.
I also have the following in my ~/.config/zathura/zathurarc file:
set window-height 3000
set window-width 3000
This starts the window "maximized".
Note that xdotool only works on X11, and I'm not aware of alternatives for Wayland.
This could be much easier if Zathura allowed the feedkeys shortcut function to be used outside of map, or any of the other shortcut functions for that matter.
Hope this helps someone else.