From what you said I understand that you have to monitor the size of a folder.

Check two examples:

Monitoring changes in a directory
http://www.news2news.com/vfp/?example=117&function=168

This code is based on a timer calling FindFirstChangeNotification API function.

Monitoring changes occurring within a directory (Win2K/XP)
http://www.news2news.com/vfp/?example=400

This code stays in a constant loop waiting for a change within a directory: any file name, attributes, size etc. depending on FILE_NOTIFY_CHANGE flags set. Using of ReadDirectoryChangesW API function is limited in VFP code unless you decide to call this function from FLL.

* * *
Once you have a notification that a file changed within the directory, you may run a procedure calculating the total size of the files.

#DEFINE FILE_NOTIFY_CHANGE_FILE_NAME      1
#DEFINE FILE_NOTIFY_CHANGE_DIR_NAME       2
#DEFINE FILE_NOTIFY_CHANGE_ATTRIBUTES     4
#DEFINE FILE_NOTIFY_CHANGE_SIZE           8
#DEFINE FILE_NOTIFY_CHANGE_LAST_WRITE    16
#DEFINE FILE_NOTIFY_CHANGE_LAST_ACCESS   32
#DEFINE FILE_NOTIFY_CHANGE_CREATION      64
#DEFINE FILE_NOTIFY_CHANGE_SECURITY     128