Basically, use Decrypter to make a decrypted image file. It'll be around 8 gigs if it's a dual layer. Then, use DVDShrink on that image to make it half. Next, burn with Nero or whatever ISO burner you use.
Just as a warning, some people encounter crashes when they try to "Burn with Nero" from DVDShrink. I usually burn the image seperately.