I cannot follow you here, ReneB.
The audio streams have a constant bitrate, so calculating their size is simple math.
You have got the time of the movie, the bitrates of the audio streams and the size of the movie with all streams.
Since subtitles only add 10MB or less, they can be ignored.
I'm really with Gerti here. It would be a really useful feature without much coding effort.
Audio Bitrate * Time = Size of Audio Stream
Movie size - n(Audio bitrate * Time) = Video only size
Then you can add the video size and the size of the audio streams according to which streams are checked by the user.
Also, you can inform about the compression rate of the video:
(4.36GB - n(Audio bitrate * Time) ) * 100 / Video only size
You could even compare avg. bitrate of source and destination.
I would very much love to see this feature! Really! Please think it over ReneB