file size and duration

how to save file size and duration of a uploaded video file.
$this->upload->data(file_size)
must work for storing file size but it doesn't work on my server, here is the error:
Use of undefined constant file_size - assumed 'file_size'

As @arma7x points out, you need to put quotes around file_size. Keep in mind that upload->data() isn't going to work until after upload->do_upload() has been called.

@site2017 presents a reasonable method to approximate the duration. If you want more accuracy here are a couple libraries that you might find interesting.

Zedwood.com PHP Calculate Duration of MP3
jagneshchawla/audio length Mp3 class- PHP

If you are looking for a library that provides additional file info and/or is useful for many different file formats beyond mp3 check out the following.

getID3()