The inspector

On the right hand side of the window, you’ll find the inspector, which will display detailed information about whatever object you have selected in the rest of the window. You can use this to see information for libraries, albums, or photos. The following information is displayed here:

Libraries: The library’s name, version, size, and location on your computer.

The size shown is for the entire library package, including all of Photos’ own data files. This should normally match the size reported for the library in the Finder. Note that this size will be larger than the size shown in Photos itself, which only adds up the space taken up by the photos themselves.

Albums: The album’s name and description, and the number and size of the photos it contains. Note that the size calculated for an album only includes the size of the original photos/videos in the album, and doesn’t take into account extra data like Live Photo videos, edited versions of photos, thumbnails, and other caches.

Photos: All of the photo’s attribute: name, date, favorite, keywords, caption, place, kind, and size. The inspector pane will also let you edit photos’ titles, keywords, and captions.