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:
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.