image/repart: add image.repart.imageSize for --size

This allows users to specify their desired total size of the generated
disk image. This is useful to e.g. match the size of a target medium to
have a appropriately sized filesystems even before first boot.

It's a noop for the default value of "auto".

nixos/modules/image/repart-image.nix

@@ -37,6 +37,7 @@
   split,
   seed,
   definitionsDirectory,
+  imageSize ? "auto",
   sectorSize,
   mkfsEnv ? { },
   createEmpty ? true,
@@ -170,7 +171,7 @@ stdenvNoCC.mkDerivation (
     systemdRepartFlags = [
       "--architecture=${systemdArch}"
       "--dry-run=no"
-      "--size=auto"
+      "--size=${imageSize}"
       "--definitions=${finalAttrs.finalRepartDefinitions}"
       "--split=${lib.boolToString split}"
       "--json=pretty"

nixos/modules/image/repart.nix

@@ -201,6 +201,14 @@ in
       '';
     };
 
+    imageSize = lib.mkOption {
+      type = lib.types.strMatching "^([0-9]+[KMGTP]?|auto)$";
+      default = "auto";
+      example = "512G";
+      description = "Size of the produced image in bytes with optional K, M, G, T suffix,
+        or 'auto' to determine the minimal size automatically";
+    };
+
     package = lib.mkPackageOption pkgs "systemd-repart" {
       # We use buildPackages so that repart images are built with the build
       # platform's systemd, allowing for cross-compiled systems to work.
@@ -415,6 +423,7 @@ in
             compression
             split
             seed
+            imageSize
             sectorSize
             finalPartitions
             ;