FTPSync by itself does not impose limits on file size to be transferred. However, due to firewalls prematurely timing out FTP sessions, it's possible that your specific setup may impose a file size limit, which may be as low as several MB and as high as several GB. Usual symptoms are:
- file is transferred completely or almost completely (byte count gets close to file size),
- when transfer should end, FTPSync "hangs" for a minute or two,
- an error (426 Transfer aborted or similar error) is displayed instead of successful transfer.
While most FTP clients include various workarounds to combat this problem, there was non in FTPSync so far. The first one, a TCP/IP Keep Alive on control connection has been added in FTPSync 2.8c. I will continue to investigate further possibilities and implement them in FTPSync in future versions.