Google image search has offered the ability to see image of different sizes using the selectbox that lets you pick "small", "medium", etc. But now Google's letting you get a lot more specific on the sizes of images that you are searching for. Once you perform an image search on Google, click the "Advanced Image Search" link which is just to the right of the search box.

Once you do that, you get a bunch of advanced options for image search. The newest option on this page is one that allows you to search for images of exact height and/or width.