Displaying the filefield file's size
Are you talking about getting the actual filesize or just formatting the size?

If the first, you can add a relationship to the {files} table in your view so that you can pull in the filesize column.  IIRC, you add a relationship to the fid and then you will see File: Size in the fields.  This will not work, though, if you manually overwrite the files via SFTP, etc.

You are getting the error because of the difference between the web path and the actual filesystem path.