You can split the file into small pieces to make it easier to distribute, you compression is compression, you're not going to make the total size smaller.
You have a 10gb file. You want it smaller, you you put it into a RAR archive and it becomes 9.5gb. That's file compression, and you cannot get better then the maximum, whatever it may be.
Now, if you wanted this file to fit onto a DVD, you could set WinRar to take the 9.5gb archive, and split it into 10 95mb pieces. Fill up one disk, and put the remaining files on the other. That's another way to go about it.