I haven't really thought about how much space a movie might be but I bet it is in the 10-20GB size if the movies are full blown HD.
They will be large for sure so expect most of your computer hard drive space being consumed by your movies...
Definitely would require an external hard drive setup. I know my daughters TIVO DMR stores many 1 hour TV Shows. I think I remember it being around 75 hours of recorded TV shows and maybe the hard drive is a 600GB size hard drive???? These are probably standard definitions format shows...
Just more things to think about getting all of your disc stored on a hard drive...
Roy Ken