In the lower left corner of the window, you’ll find the information pane, 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 a library, album, or individual photo. 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. 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.
Photos: all of the photo’s attributes (name, date, favorite, keywords, description, place, kind, and size) as well as the full paths to both the original and edited versions of the image. Note that if you’re view your iCloud library and the selected photo hasn’t been downloaded locally yet, then no path will be shown for that photo.