Login to your host and do the following:
virsh domblklist myVM
qemu-img info /var/lib/libvirt/images/myVM.qcow2
qemu-img resize /var/lib/libvirt/images/myVM.qcow2 +16G
It is not required to check the current disk size, but it is good to know. You can take a look afterwards if the resize worked.