A 3rd party file manager like Directory Opus shows you the size of the files you selected.
Don't take this the wrong way, but here's a taught. With the amount of file management you need to do, it might be useful to start looking into one of those. Spend the cash and automatize what you can within that software. You have a GUI to work with, In case of Directory Opus, also an extensive manual, the batch scripts you already have can be implemented as a new button in DO and while your automation is busy, you'll have a progress bar to see how far along you are.
Windows Explorer (in any version of Windows) is not a good fit when you need to manage lots of files. It works more or less for tiny to small file-sets. But it is a time sink and RSI inducer when you try to use it with big file-sets. So falling back to scripting appears the next best thing to do. And it is.
However, you seem to be depending with regards to scripting batch files, so it might be handier for you to start considering 3rd party file managers, like: Directory Opus, Total Commander, XYplorer, xplorer2, Double Commander, Free Commander, Altap Salamander, etc. (this is a list of managers I tried and is in order of my preference).
for a list of 26 3rd party file managers. The fact that there are so many, indicates that Windows Explorer is flawed (deeply).
While scripting is an excellent idea for menial file management tasks, you must either learn (and practice) scripting or get a tool for the job. That way, it is much easier to execute your ideas on how you want your files to be managed the moment you have a new idea about how to do that.
The time being lost by writing down your idea, making the request here at DC, waiting for a response and then testing the response, could have been spend on learning how to script or doing the job with a 3rd party file manager.
On a side note: I was not aware of the existence from Multi Commander
. I am taking a look at it and am pleased. The fact that is portable, scores high with me, the Directory Opus 4 "look" (Amiga home computers owners know what I'm about) scores high too and you can edit these buttons extensively, but also the main menu and context menu as well. I start to think it should take the place xplorer2 in my list...maybe even one higher.