My settings are like this:
In my case, the problem was not on the host (SPICE client) size, but on the guest.
It seems, that KDE for some reason forced some fixed display resolution and declined any changes that were sent via SPICE channel. Manually selecting current window resolution in the guest’s Display Configuration helped.
If you’re interested in how I got to this solution, here’s how. I accidentally spotted that when I change the size of the virt-manager console window, the opened Display Configuration inside guest VM shows an asterisk symbol in its title, like there were some unsaved changes. Comparing the “Resolution” drop box contents before and after window size change, I found out that one of the list items actually changes its value accordingly to the virt-manager console dimensions, but KDE for some reason just ignores it. After I selected that particular resolution item, everything started working again.
So, if in your case you don’t have this item that tracks correct window size resolution in the list, you may have a problem with something else (SPICE channel spice-vdagent etc.).
P.S. In X11 session automatic resizing worked well all the time, I experienced problems only in Wayland session.