--- a/testing/marionette/driver.js
+++ b/testing/marionette/driver.js
@@ -2400,18 +2400,20 @@ GeckoDriver.prototype.setScreenOrientati
  * Get the size of the browser window currently in focus.
  *
  * Will return the current browser window size in pixels. Refers to
  * window outerWidth and outerHeight values, which include scroll bars,
  * title bars, etc.
  */
 GeckoDriver.prototype.getWindowSize = function (cmd, resp) {
   let win = this.getCurrentWindow();
-  resp.body.width = win.outerWidth;
-  resp.body.height = win.outerHeight;
+  return {
+    width: win.outerWidth,
+    height: win.outerHeight,
+  };
 };
 
 /**
  * Set the size of the browser window currently in focus.
  *
  * Not supported on B2G. The supplied width and height values refer to
  * the window outerWidth and outerHeight values, which include scroll
  * bars, title bars, etc.