Check this thread for example ftp backup script-
http://cpug.org/forums/showthread.php?t=1375.
Or you can use scp for backup. See parametres "backup" commands from command line.
About tftp. Size limit depends on tftp server, for example next server (found by google
http://www.freefunfiles.com/software...ftp-turbo.html) doesn't have 32MB file size limitation.