To use the progress monitor, you need to implement FTPProgressMonitor in a class of your own, e.g. MyFTPProgressMonitor.
Get the size, and then pass it into MyFTPProgressMonitor via the constructor, e.g.
FTPProgressMonitor monitor = new MyFTPProgressMonitor(size);
Call ftp.setProgressMonitor (monitor, interval) to set it up before you do the transfer.
When the bytesTransferred(long count) method is called by the FTPClient, you can work out the percentage transferred each time.
How would I go about passing the size into the monitor?