diff --git a/cmd/metal-api/internal/service/size-service.go b/cmd/metal-api/internal/service/size-service.go
index cf5510a1..1955c725 100644
--- a/cmd/metal-api/internal/service/size-service.go
+++ b/cmd/metal-api/internal/service/size-service.go
@@ -164,7 +164,7 @@ func (r *sizeResource) webService() *restful.WebService {
 		Operation("updateSizeReservation").
 		Doc("updates a size reservation. if the size reservation was changed since this one was read, a conflict is returned").
 		Metadata(restfulspec.KeyOpenAPITags, tags).
-		Reads(v1.SizeUpdateRequest{}).
+		Reads(v1.SizeReservationUpdateRequest{}).
 		Returns(http.StatusOK, "OK", v1.SizeReservationResponse{}).
 		Returns(http.StatusConflict, "Conflict", httperrors.HTTPErrorResponse{}).
 		DefaultReturns("Error", httperrors.HTTPErrorResponse{}))
diff --git a/spec/metal-api.json b/spec/metal-api.json
index 0d5caf7d..01889dae 100644
--- a/spec/metal-api.json
+++ b/spec/metal-api.json
@@ -4817,6 +4817,46 @@
         "sizeid"
       ]
     },
+    "v1.SizeReservationUpdateRequest": {
+      "properties": {
+        "amount": {
+          "description": "the amount of reservations of this size reservation",
+          "format": "int32",
+          "type": "integer"
+        },
+        "description": {
+          "description": "a description for this entity",
+          "type": "string"
+        },
+        "id": {
+          "description": "the unique ID of this entity",
+          "type": "string"
+        },
+        "labels": {
+          "additionalProperties": {
+            "type": "string"
+          },
+          "description": "free labels associated with this size reservation.",
+          "type": "object"
+        },
+        "name": {
+          "description": "a readable name for this entity",
+          "type": "string"
+        },
+        "partitionid": {
+          "description": "the partition id of this size reservation",
+          "items": {
+            "type": "string"
+          },
+          "type": "array"
+        }
+      },
+      "required": [
+        "amount",
+        "id",
+        "partitionid"
+      ]
+    },
     "v1.SizeReservationUsageResponse": {
       "properties": {
         "amount": {
@@ -9236,7 +9276,7 @@
             "name": "body",
             "required": true,
             "schema": {
-              "$ref": "#/definitions/v1.SizeUpdateRequest"
+              "$ref": "#/definitions/v1.SizeReservationUpdateRequest"
             }
           }
         ],