RZL_READ_DIR_LOCAL
Export to function the server address,
The function returns the files.
GUI_GET_FILE_INFO
Export to function the files (addres + file name), example: folder/file.txt
The function returns the size
The base will grow regarding the files and the size... if you have a day average you can foresee.
If you don't have a average and the user can upload any number any time, create a program that calculate the size and sends a message (like e-mail) after the server have X megas. And you can create a job to this program to send automaticaly an alert e-mail.