Thanks for your input.
All I do is call tar to create a file.tar.gz for the directory I wan to backup.
Then the file is copied to another computer and backed into DVD. My problem is that the other computer creates 650MB zip files, and when this tar file became 700MB it was not getting included in the backup. With split I can make sure it does.
I do not know how to check the future size of a tar file before creating it.