1. Install guestfs-tools

$ apt-get update
$ apt-get install guestfs-tools

2. Download an image

Download an image from https://cdimage.debian.org/images/cloud/.

3. List partitions

Check the image disk partitions to see which partition needs resizing.

$ virt-filesystems --long -h --all -a debian-12-nocloud-amd64.qcow2

4. Create output container

$ qemu-img create -f qcow2 -o preallocation=metadata debian-12-nocloud-amd64-resized.qcow2 20G

5. Generate resized image

$ virt-resize --expand /dev/sda1 debian-12-nocloud-amd64.qcow2 debian-12-nocloud-amd64-resized.qcow2

Conclusion

You can now open debian-12-nocloud-amd64-resized.qcow2 with a virtual machine program and the disk size will be 20G.

References

virt-resize manpage

$ man virt-resize