It is left to the DM to resize after being informed by libvirt there has been a change. Many don’t.
Poor design decision IMHO.
xrandr --output Virtual-1 --auto
Will resize to the window size you have chosen. The Virtual-1 part might need adjusting. You can find out by using
on it’s own. In the case of a Manjero VM I just installed
$ xrandr Screen 0: minimum 320 x 200, current 1256 x 869, maximum 8192 x 8192 Virtual-1 connected primary 1256x869+0+0 (normal left inverted right x axis y axis) 0mm x 0mm 1256x869 59.98*+ 2560x1600 59.99 59.97 ....................................
So it’s Virtual-1.
Make a script and link it to a hotkey and it’s less of an annoyance.
Note: spice-vdagent may need to be installed and running for this to work ?