Qemu/virt-viewer console window with full-screen in X

[mister knister]

I would like to build a lean KVM/Qemu Hypervisor environment with a Debian 11 base.
Therefore I installed xserver-xorg, qemu-system-x86 and virt-viewer. (So no desktop environment - just X.)
Imported VMs run fine with calls like:

  xinit /usr/bin/qemu-system-x86_64 -hda /home/user/vms/myvm.qcow2 -m 2048 -vga virtio -full-screen

X opens, vm starts, virt-viewer? console starts and vm screen output is updated.
However the console window is opened only with about a quarter of the available X screen size (640x505 of 1920x1080)

Question: Is there a way to tell Qemu/virt-viewer to open its window as full screen within X?

The "-full-screen" option in the Qemu call above only tells the viewer to hide the menu bar.
Additionaly qemu-system reports, that "xinit -g XxY+0+0 ..." geometry calls are also not supported.