docs: add diskSize flake example

README.md

@@ -116,10 +116,63 @@ on the generated NixOS build.
 - vmware
 
 Example (20GB disk):
-```
+
+```bash
 nixos-generate -c <your_config.nix> -f <format> --disk-size 20480
 ```
 
+To set the disk size in `flake.nix`, set `diskSize` in the `specialArgs` argument of the `nixosGenerate` function.
+
+```nix
+{
+  inputs = {
+    nixpkgs.url = "github:NixOS/nixpkgs/nixos-23.11";
+    nixos-generators = {
+      url = "github:nix-community/nixos-generators";
+      inputs.nixpkgs.follows = "nixpkgs";
+    };
+    xc = {
+      url = "github:joerdav/xc";
+      inputs.nixpkgs.follows = "nixpkgs";
+    };
+  };
+
+  outputs = { nixpkgs, nixos-generators, xc, ... }:
+    let
+      pkgsForSystem = system: import nixpkgs {
+        inherit system;
+        overlays = [
+          (final: prev: { xc = xc.packages.${system}.xc; })
+        ];
+      };
+      allVMs = [ "x86_64-linux" "aarch64-linux" ];
+      forAllVMs = f: nixpkgs.lib.genAttrs allVMs (system: f {
+        inherit system;
+        pkgs = pkgsForSystem system;
+      });
+    in
+    {
+      packages = forAllVMs ({ system, pkgs }: {
+        vm = nixos-generators.nixosGenerate {
+          system = system;
+          specialArgs = {
+            pkgs = pkgs;
+            diskSize = 20 * 1024;
+          };
+          modules = [
+            # Pin nixpkgs to the flake input, so that the packages installed
+            # come from the flake inputs.nixpkgs.url.
+            ({ ... }: { nix.registry.nixpkgs.flake = nixpkgs; })
+            # Apply the rest of the config.
+            ./configuration.nix
+          ];
+          format = "raw";
+        };
+      });
+    };
+}
+```
+
 ## Cross Compiling
 
 To cross compile nixos images for other architectures you have to configure