Now, After the new Google search layout, Google has removed the left side bar from the search results page. All search options are now added to the top with “Search By Size” option. But “Exact Size” search option is not there. You are forced to select given size options from the list.Although Google has removed this option, you can still search images by exact size. You can do this by using “imagesize” search operator in the query.
Once you press enter to search images, Google will remove the imagesearch operator from the query and show image results of the size mentioned by you.
This operator also works on mobile version of Google.