Google Image lets you search for extra large, large, medium and small size images. Desktop wallpapers however lie between extra large and large size depending on the monitor size.
Amongst the visible options, no specific size is given. Microsoft Search Engine Bing scores above this facility for search of wallpaper size images. On the right side of Bing result page, you can find an option to search for wallpaper images. This option automatically calculates desktop dimensions and return exact size images.
A simple trick to search for Desktop size or specific size in Google images is to append your monitor resolution to search word. For example, to search for flower wallpapers for a monitor resolution of 800 by 600, enter flower 800×600 in search box [try ] or barack obama 1280X800 [try] for wallpaper of size 1280 pixels by 800 pixels.
And you can always go to advance search for other options including exact size, file types and automatic detection of desktop size.