Actually, looks like I under-estimated by half. One of the random VMs I run has a command-line 1361 bytes long. Here it is sanitized:
qemu-system-x86_64 -enable-kvm -name hostname.domain -m 4096 -smp 2 -pidfile /var/run/ganeti/kvm-hypervisor/pid/hostname.domain-device virtio-balloon -daemonize -D /var/log/ganeti/kvm/hostname.domain.com.log -machine pc-i440fx-bionic -monitor unix:/var/run/ganeti/kvm-hypervisor/ctrl/hostname.domain.com.monitor,server,nowait -serial unix:/var/run/ganeti/kvm-hypervisor/ctrl/hostname.domain.com.serial,server,nowait -usb -usbdevicetablet -vnc 127.0.0.1:5156 -chroot /var/run/ganeti/kvm-hypervisor/chroot/hostname.domain.com -uuid 3d9ea1c9-33ce-4e06-81d3-a97005b89e4b -netdev type=tap,id=nic-19f31fbd-f5de-4b57,fd=9 -device virtio-net-pci,bus=pci.0,id=nic-19f31fbd-f5de-4b57,addr=0xd,netdev=nic-19f31fbd-f5de-4b57,mac=aa:00:00:XX:XX:XX -netdev type=tap,id=nic-0e7f0c3a-e19b-4ae2,fd=12 -device virtio-net-pci,bus=pci.0,id=nic-0e7f0c3a-e19b-4ae2,addr=0xe,netdev=nic-0e7f0c3a-e19b-4ae2,mac=aa:00:00:XX:XX:XX -qmp unix:/var/run/ganeti/kvm-hypervisor/ctrl/hostname.domain.com.qmp,server,nowait -qmp unix:/var/run/ganeti/kvm-hypervisor/ctrl/hostname.domain.com.kvmd,server,nowait -bootc -device virtio-blk-pci,bus=pci.0,id=disk-066d688d-9f5a-4ca5,addr=0xc,drive=disk-066d688d-9f5a-4ca5 -drivefile=/var/run/ganeti/instance-disks/hostname.domain.com:0,format=raw,if=none,id=disk-066d688d-9f5a-4ca5 -S -runas kvm37