you can do this using agent and parameter File.Size with recursive option set to on. For example, to monitor total size of all files under /home/user, add the following DCI:

File.Size(/home/user,*,1)

For reference, full description of File.Size is below:

File.Size(path, pattern, recursive, size, age)
where
    path    : path to directory or file to check
    pattern : pattern for file name matching
    recursive : recursion flag; if set to 1 or true, agent will scan subdirectories
    size      : size filter; if < 0, only files with size less than abs(value) will match;
                          if > 0, only files with size greater than value will match
    age      : age filter; if < 0, only files created after now - abs(value) will match;
                          if > 0, only files created before now - value will match

all arguments except path are optional.

Same syntax is for File.Count, which will return number of files instead of size.

Best regards,
Victor

You can only monitor file systems, not separate files, so if you want to monitor a directory, it has to be mounted (the actual directory needs to be mounted), not just a part of a mounted FS.