add dkimKeyBits configuration

default.nix

@@ -422,6 +422,19 @@ in
       '';
     };
 
+    dkimKeyBits = mkOption {
+        type = types.int;
+        default = 1024;
+        description = ''
+            How many bits in generated DKIM keys. RFC6376 advises minimum 1024-bit keys.
+
+            If you have already deployed a key with a different number of bits than specified
+            here, then you should use a different selector (dkimSelector). In order to get
+            this package to generate a key with the new number of bits, you will either have to
+            change the selector or delete the old key file.
+        '';
+    };
+
     debug = mkOption {
       type = types.bool;
       default = false;