Google Images used to have a very useful "search by size" option in advanced search to help you find logos, wallpapers and other images on the Internet by their exact size (or resolution).
For example, you could limit your search to horizontal photos to image files at least 10 Megapixels in size.
Alternatively, if you use Google Image Search to find desktop wallpapers, you could set the image resolution to 1920 × 1080 pixels, and Google would return large images with only those exact dimensions.
The "exact size" search option is no longer available in Google Image Search, but you can still specify the size of the images in your searches using a search algorithm command.
Open the address images.google.com and enter your search terms as you always did. Then add imageize: LENGTH to your query and press Enter. Google will read the additional command in the query, remove it, but the results will only display images that match the specified size.
This resize search also works in the mobile version of Google. So you can use this simple trick to find the perfect wallpaper size for your phone.