Hi -
1 - No, you can't see the size of the optimised file, and in any case it is not that simple. Space on your drive is taken by preview and thumbnail versions of the image, as well as the database storing all the organisational structure of your library, and the edits to your photos.
2 - When you edit in photos, a new file is not created. The database simply stores the "instructions" for the edits so a new file can be created dynamically from the original by applying the edits whenever needed (eg for further editing, printing, or export). Yes a copy of the original is downloaded so you can see all the detail while you are editing. After you have finished editing, then (eventually) the copy of the origninal on your mac will be removed, but maybe not until the space is needed. The exact defintion of when it is removed is not published.