There is not "free space inside a folder". Folders can be filled with data until the disk is full or if a disk quota for a specific user is reached. You can check the available disk size by:
FileObj = java.io.File(Folder);
free_bytes = FileObj.getFreeSpace;
total_bytes = FileObj.getTotalSpace;
usable_bytes = FileObj.getUsableSpace;
Note that filling a disk to more than 99% (95% is better) might cause a loss of data. If the disk is exhausted, the behaviour is not well defined.
