The source of ROTK was 1080p, I re-encoded it down to 720p @ 6000kbs with 2ch AAC and 5.1 AC3 - Quality is awsome
I was expecting around 8gb to be honest as the film is over 4 hours long!! I was going to re-encode with a lower bitrate to get a smaller file size but it plays fine and looks great so I decided to stick with it.
Fellowship is 7-8gb
Two Towers is 7gb
6000 Might be overkill, I duno? Saying that I have just encoded The Dark Night @ 6000 with 5.1 AC3 and that is under 4gb