The problem with sizes is that, unless you want the exact number of bytes, the results are a bit confusing. In fact I have a post about it with no replies so far; I didn't try anything after that.
I'll consider some option for that.
The main issue with that approach is that there'd be no way to know what the command is doing, or to stop it. This is more noticeable when it's used with lots of files. A better option might be adding a delay before showing the dialog, so it won't pop up if the operation finishes quickly.
Actually, this script was actually a bit forgotten; I'll update and polish it when I have more time.
(One drawback for that with this and my other scripts is the lack of feedback, probably because they're not used much or something like that. That also kind of puts me off releasing new versions or other scripts.)