We'll create fresh WordPress site with Display Registered Image Dimensions installed. You have 20 minutes to test the plugin after that site we'll be deleted.
This plugin will display the images defined using add_image_size in function.php and from installed plugins. Values are displayed in the Media Settings screen.
You can see how images are https://codex.wordpress.org/Function_Reference/add_image_size
This plugin is based on the plugin: “Image sizes in admin dashboard!” by mansoormunib