The inclusion of an optional file size column when viewing a library in List View, would allow for quick and easy management of larger items in your library.
Resolution & Bitrate helps to narrow this down, but nowhere near as useful as actual file/folder size on disk in terms of managing server disk space.
Strongly agree that this would be a valuable feature enhancement. Plex already has file management capabilities since you can delete media directly from the client-side viewer, and enumerating filesizes is very computationally-simple. Files in the Plex library have already been parsed (computationally-complex) for all kinds of parameters, including duration (for videos), so it should be extremely easy to add “filesize” as a sorting option to the Plex player/client GUI.