Most image search engines offers an option to filter the results by size, but you can only choose between small, medium and large images. Google Image Search has an undocumented parameter that lets you specify the exact dimension of the results:
imagesize:WIDTHxHEIGHT.
Here's an example: [imagesize:640x480 muffin] finds 640px x 480px images related to muffins.
The operator could be useful if you need to find wallpapers for your computer, logos, illustrations for a school project or any other images that have standard sizes.
{ Thanks, Michael Garmahis. }