As far as I know there is no way to directly derive the length of an ebook file without actually opening it and parsing its contents. The file size can be a very rough indication, but that can be heavily affected by both how text has been formatted, and also whether there are embedded graphics.
I do not recall any interest expressed by any of the calibre developers in having such a feature. This matters as since calibre is open source, features only get added to calibre if someone is interested enough to work on them. Of course I could be wrong there and your post could spark off interest.