Unfortunately Monarch does not provide a function that will print a report file size.
I suggest you use the DOS dir command and redirect file names and sizes to a file (e.g. Dir C:\temp\* > file.txt). Then use Monarch desktop and bring file.txt as an input, create two columns, filename, and file size. Export the table to another file. Then open your original report, use the file() function to capture file names. Bring the exported file that contains filename and file size as an external lookup, link file names, and merge file size.