The “exact size” search option is no longer available in Google Image Search but you can still limit your image searches to a particular size by using the secret imagesize search operator in the query itself.
Go to images.google.com and enter the search terms as before. Then append imagesize:WIDTHxHEIGHT to your query and hit Enter. Google Images will remove the operator from the query but the results will only display images that match the specified size.
The search by size operators works on the mobile version of Google as well so you may use the simple trick to find that perfect size wallpaper for your phone.