I wanted to find some 600×800 images for the Kindle and learned that Google Images doesn’t let you perform this kind of search now. However, there’s a small hack that lets you do exactly the same. Append the following text to the end of the Google Image search URL:
This will return only the images that match the exact size. Like in this example. You may replace the 600 (width) and 800 (height) with whatever dimensions you need.