nixos-mailserver/nixops/vbox.nix

{
  mailserver =
    { config, pkgs, ... }:
    { deployment.targetEnv = "virtualbox";
      deployment.virtualbox.memorySize = 1024; # megabytes
      deployment.virtualbox.vcpu = 2; # number of cpus
      deployment.virtualbox.headless = true;
    };
}