By default WordPress doesn’t tell you the attachment file sizes like images and other media you upload to WordPress using ‘Add Media’ button. This tutorial is on new WordPress plugin Display File Sizes, the plugin simply adds a file size field in the Save meta box on attachment edit screens and makes it easy for you to identify large images/files.
Installation & Usage: Simply install and activate Display File Sizes and it automatically displays the attachment’s file size on that attachment’s edit screen.