Google images has very huge source of images pertaining to all terms. You can search any type of images on google images by exact size. Whether you are searching images for your blog post or you are searching a image of particular keywords and having specific size which you want to use as wallpaper on your computer.
So you can use simple google advanced images search trick which will help you in searching google images by exact size.
Although there are google advanced search options called search tools available in google images search options as shown in below image which can be used to search google images as per your options obeying different parameters as selected by you. But these tools do not provide option to search google images by exact size.
For this purpose, here is simple trick which you can use to search google images by exact size as per your choice.
For example if you want to search google image for Windows 8 wallpapers having size 740 pixel x 520 pixels. Just go to google images and make a search for terms imagesize:840×520 windows 8 wallpaper
Google will display all the images having that particular size and pertaining to your keywords. In this search query, Do not write image size dimension units like pixels. Just write imagesize :widthxheight numeric values. By default, Google will take pixels as unit for these values.
So enjoy this simple but effective Google advanced image search trick to search for your favorite Google images by exact size pertaining to your keywords.