[workaround]
I too have struggled with this. I find that if I use spicy (spice-client-gtk) and this command
xrandr --output Virtual-0 --auto
it does do the resize right if vdagent is installed on the host and the guest and the qxl driver are installed. I have taken to putting a shortcut to that command as an item in my menu or panel. (making sure that that thing is in the upper left corner so I can always get to it.)
But I can't see any event to watch to automatically do the resize. Windows 7 and Windows 10 guests do it perfectly. Why not linux????