Window Watcher for Chrome it's a extension that shows the size of the window without working directly with the inspector or other developer tools.
Just click here to download extension from Chrome Web Store. It's free!
Just download window-watcher-for-chrome folder and go to chrome://extensions and then activate the developer mode on the top right of the page. Click into Load unpacked extension and indicate "window-watcher-for-chrome" folder.
Now, when you load or resize the page, you see the size on the top right.
In the next versions it will be possible to customize all parameters of the extension, such as speed, time or color. As you like!
2.0.1 (updated on 21 Sep 2017)
You can contact me via Twitter @sergiobarriel