The concept of window-in-window is a relatively new one to me, and though I saw it originally applied to wide angle photography (another iteration I'd like to try) it's perhaps a bit easier to achieve with a longer lens that more readily distorts relative size. If I had to estimate, I'd say that space in the fence I was shooting through is about a hand breadth wide and two tall. Once again it's all about emphasis though, because it would have been just as easy to shoot around the fence altogether to get a similar (though I would say less interesting) resulting image.
EXIF data:
1/400, f/8, ISO 200
200mm focal length
1/400, f/8, ISO 200
200mm focal length