So let me explain it a bit. You pass a path to a folder. The plugin runs through all folders and subfolders and sums up the file sizes. This is very quick and works well. We have a few options like limiting the recursion level, ignoring content of hidden folders and how to treat bundles.
Now the calculation gives us a lot of values, so the function returns a list of values. We don't want you to call this function several times just to know how many files are there and how many bytes those take. So we give you all values from one call. You pick the values you need. And you see on the documentation, we have a lot of values for data fork vs. resource fork, logical vs. physical size and visible vs. hidden vs. all files.
The function will be available next week with 6.2pr1. if you like to test now, please email us.