MP3tag does not know anything about other files in a folder, so the size of a folder cannot be read from a single field (which does not exist).
You could create an export that counts the size of all treated files
see the help: https://help.mp3tag.de/options_export.html
but that does not mean that this is the size of the folder as there could be other files in that folder that are not treated by MP3tag - and therefore not counted.
Once you created the export you have to find a way to get the size value somehow into the folder name ...
And then the data does not update automatically if you add or remove files or add e.g. a cover.
So I wonder where the benefit would be.