The FileManager only reports file sizes given to it by the Operating System so directories are not the total of all files and sub directories. So a directory might have 1GB worth of files, but the filemanager would report the directory as only 1.0K
I only know how to get sizes in Internet Explorer. Once logged in you right click and go to properties. Basically like looking up file/directory sizes on windows like normal.
Flash-FXP might have something similar. Try highlighting all files and folders that you want to include in the tally and see if there's any information/properties page for them, that might give you the combined usage.