I don't quite understand the question. The exact size of the image depends on the size you bought from iStockPhoto.
But if you want to know how large real polorids were well: there were lots and lots of sizes but the most popular was 3¼" x 4¼" (including border). That is the "standard" size.
Thanks for the reply. No, the question has nothing to do with the image resolution in iStockphoto. Yes, I'm aware of the different sizes of Polaroids. I just want to know the real-life size of the image shown in that link. What's the dimension of both the white portion of the Polaroid and the image area.